Travel Purse

I use the Baggalini Everywhere bag. It could easily hold your water bottle.

An anti-theft bag plus a 26 oz filtering water bottle is going to be pretty heavy during a day of sight seeing.

I used to travel with the LeSportSac deluxe everyday crossbody bag. xhttps://www.lesportsac.com/products/deluxe-everyday-bag-beige-7507-u202 It is super lightweight, has several pockets for smaller items, and at roughly 9 x 14 inches, should be fine holding your water bottle, assuming it is leakproof on its side? In addition, it is expandable, in case you need to add an umbrella or other item in addition to the items you want to carry. There are quite a few color and print options on eBay at much less than the current retail price. I would suggest (from experience) getting one that does not have a black or other dark color inside, for better visibility.
Another possibility if you want a narrower but deeper bag, is the Kipling Alvar in the medium size.
https://www.kipling-usa.com/alvar-crossbody-bag/HB4061.html?dwvar_HB4061_color=666#uuid=2dd3717bca475c4b2a83468b32
Likewise, there are good options and prices on eBay.

I really like having bags that can be washed occasionally and these are very good quality, durable and lightweight options.

So why do you think you need a secure bag? Are you afraid someone is going to slash it? The sellers of these purses want you to think this happens every day but trying to find someone who’s actually seen it occur is pretty hard. It’s just so rare that you don’t need to protect yourself against it.

Are you afraid somebody’s going to rip your purse off your body? well, then what you should do is connect it to your body an unbreakable cord because that way they could drag you in a traffic and you could be run over by bus. And yes, I saw a snatch grab in Naples. Had that woman had her Wallet attached to her body with an unbreakable cord the guy on the motorcycle would have drug her into oncoming traffic, but she would have had her wallet . I’ve never used an unbreakable cord and watching that scene I knew why. I’d rather give up my possessions, then be run over by Naples traffic

I do admire the manufacturers of these bags, they have managed to create an entire industry of fear so that they can make a lot of money. It’s great marketing, but it’s not really required imho.

I’m a 60 something year-old woman I go to Europe at least once or twice a year. ( once for vacation and sometimes the second trip for business.) I take one or two purses. One is a leather Crossbody I use at home. The other is a Longchamp. Le Pilage. I come home every year with all of my possessions. I do have a back up plan, in the hotel safe or somewhere hidden is an extra credit card, my debit card, etc. I could recover in about five minutes.

I use a Tom Bin Medium Cafe Bag. It’s not a special security purse but it’s a crossbody and it fits what I need for the day, including a water bottle.

Thanks to all of you. All of you had great responses and a lot of them will make me rethink my original plan. I really appreciate you taking the time to respond.

That's great you got helpful advice! I was going to add what I do, but also ask about how you tend to travel when far from home.
My aluminum water bottle is only 12 ounces and thin. If you need and want to have the large one you like best, consider this configuration instead of one magic bag to do it all:
I like to use the stretchy jersey fabric waist bands with large pockets. I can easily fit my passport (in a snack size baggie) as well as backup cards, local currency, etc. I find them fairly easy to access if needed ( rarely). This is for days with lots of public transportation, crowds, etc.
I also use a Tom Binh pocket crossbody that's just right for my phone and a teensy card holder/change holder (bathrooms) and hand sanitizer. I have the phone tethered to the inside of my bag so I don't drop it over a waterfall. The purse is under my coat or cardigan style sweater along my side and not visible.
Then, I use an old museum pocket tote (it zips into itself to make a small square) for the water bottle and etc. In that way you can also stuff in your jacket and bigger daily needs.
It's a different way of looking at how you carry what you need? Have a wonderful trip!

I carry a 32 oz Camelbak, and I have a Baggallini that holds it - this is the closest I could find. https://www.baggallini.com/securtex-anti-theft-free-time-crossbody-bag/AFT464-BGR-B0220-NS.html

(and you'll pry my 32 oz Camelbak from my cold dead hands, btw).

Thanks again for the suggestion. I have a lot to look through but they are all great.

I just bought a Sherpani Vale crossbody bag and I absolutely love it! It folds over to make a cute, small purse or expands into a decent day tote. It holds my 64oz water bottle, a long rain parka and all the odds and ends I need without being too large and cumbersome. It’s perfect. I got the solid black one on sale from another store though, I can’t remember which.
https://sherpani.com/products/vale-at-3?variant=32511083774049

Plus, it has several anti-theft features for peace of mind.

Thank you very much - this one looks very promising! :)

You seem committed to the 26 oz. Britta Bottle. How about just buying some water at destination with a screw down lid? Grocery shopping is fun.

My concern is that I had some surgery that requires me to drink a ton of water every day. I'd rather sightsee than have to stop a lot to buy water. I'm not looking forward to lugging this bottle around, but I know from experience how often I need to drink everyday. From reading all of your comments, it does seem that just stopping makes more sense....I'm just trying to balance it all.

If my beverage bottle has a good seal - i.e. screw on lid - I carry it inside my bag rather than in the side pocket.

I think the 26 oz bottle is going to be bulky to pack. Grocery shopping in foreign countries can be a fun travel experience of its own. Quick, also. There are 1 liter bottles (and smaller) of water with screw lids that can be purchased. I would buy some of these for the hotel room and carry what I need for the day.
Then, you can just throw out the bottles if recycling them is a hassle.

When you travel - there is going to be some “down” time. Down time is a good thing. Break at hotel room in mid-day, haircut, laundry, reorganize when original plans don’t work, grocery store visit, casual stroll, bank, etc.. These times can be an enjoyable part of your trip. So, don’t think you are just going to be full-on, hard press sightseeing all the time. Enjoy these other moments as well. You may meet some locals, have some unplanned fun experience, discover something interesting that you didn’t know, etc..
I have some fun memories from my unplanned, non-sightseeing time.

@Carol,
I enjoyed your post about being dragged around by unbreakable cords. True. Good safety points. I only met one person who told me her bag was slashed open with a knife and that happened in Central America. She also had strong local ties, not a tourist. She was in an area where there was high crime and she used precautions with her money and credit cards. Gave me advice on the airplane.