How to use iphone internationally

Is your phone locked? I have ATT so I don't know what verizon does, but if your phone is locked, that means you cannot put another SIM card in. So that's your first question, hopefully someone with more knowledge of Verizon's policies will help. You can always call customer service to ask.

If your phone is not locked then you can buy a local SIM card.

If your phone is locked, you can buy a plan through Verizon. I don't know their specifics, but with ATT there is a $10/day plan (you pay for each day you use data on the phone but can use it just like you're at home, no extra limits, and if the phone is in airplane mode or data off all day then you don't pay that day). There's also a very limited plan for something like $60 and $120 per month depending on data amount, that gives you a small amount of data. This can work if you limit your data use.
For a shorter trip the $10/day plan makes sense. 7 days, $70 for the full use of your phone as at home isn't bad. But for long trips it really adds up!

So - buy a plan or just use wifi. Download all maps ahead of time at the hotel's wifi, and get good at searching out public wifi in cafes. Make sure cellular data is set to OFF or else you may end up with charges. It's not ideal but can be done.

I just purchased an iPhone XS from Verizon, and was pleasantly surprised that it has the dual SIM capability. The way it is set up is to have two lines on the phone, say one for your work number and the other for your personal number. Where it is convenient to me is to set my personal number on the "eSIM" or electronic SIM, which doesn't require an actual card, and to set my second line in the actual SIM card slot. This way, when I get to Europe, I can purchase a "tourist" SIM card and put it into the second slot to get local calls and cellular data.

This is all new and Buck Rogers cosmic stuff, so ask your provider. Verizon says that their phones are unlocked, so that shouldn't be a problem.

Verizon has an international travel plan for $10/day, and you only pay for the days that you use it. You can keep your phone on airplane mode until you need it. You can activate the travel plan through the Verizon website I believe, so you don't even need to call customer service! There is no charge if you choose not to use it, so it wouldn't hurt turning it on just in case.

Buy a SIM card in Croatia. Verizon only recently started locking new phones again (if you bought it from them) but only for a short time. Contact them ahead of time to check to see whether your phone is locked or not. If it has been more than a few months, I'd guess it is probably not locked.

With a Croatian SIM card, you'll have a Croatian phone number and won't be able to use your US number for calls or texting until you put the Verizon SIM card back in. Buy the SIM at a mobile store and set it up before you leave the store so you know it works. Keep your Verizon SIM card in a safe place. Consider taking a little "paperclip tool" to remove the SIM (the mobile store will have one, maybe they can give it to you) so you can put the Verizon SIM back in when you have left Croatia.

More info on Croatian mobile providers here:

https://prepaid-data-sim-card.fandom.com/wiki/Croatia

We've gone the route of buying sim cards in multiple countries - sometimes we had great service and other times almost no service no matter what plan we purchased in the country. We've also dealt over the years with outrageous ATT rates and the inability to set up WiFi calling without purchasing a new expensive phone. Our last two trips we purchased a cheap phone and plan from T-Mobile. They do not charge extra for data or texts and allow WiFi calling on any phone. Problem solved and it worked great in Australia, New Zealand, Austria, Slovenia and Croatia.

maps, internet, or whatsapp

Most map apps allow you to download an offline map. I use Google maps and that works fine. You cannot get step by step walking directions, but it will show you where you are and in many places you can just use driving directions and then adapt them yourself. May not work in very old parts of a town.

internet, or whatsapp: I don't know about Croatia, but everywhere I have been in the last 5 years, every restaurant and cafe had wifi.

Getting a SIM card is doable and probably a good idea but if you aren't using your phone much and have Verizon, you can use your phone and just pay an extra $10/day. If you are staying a long time and use your phone a lot then you might consider the 30 day plan for $104 which I just paid for my daughter.