Capri- Blue Grotto

Should we hire a local guide for Capri to make the most of our time or
is it unnecessary?

IMHO, a human guide is unnecessary. Buy yourself a map of the island when you arrive - you'll need it - and the Capri visitor's website has descriptions of hiking/walking opportunities: just follow the instructions and your map (we did):

https://www.capritourism.com
https://www.capri.com/en/l/hiking-and-walking-trails-capri

I'll give shout-outs to the treks to Villa Jovis/Villas of Tiberius, and Pizzolungo. The chairlift up Mt Solaro is fun. You can hike down the mountain if so inclined. :O)

https://www.capri.com/en/e/pizzolungo-2
https://www.capri.com/en/e/the-villas-of-tiberius
https://www.capri.com/en/e/monte-solaro-high-drama

We skipped the Blue Grotto and were never sorry; it's not a given anyway depending on weather and wave action, takes more time than we wanted to devote to it versus the amount of time in the thing, and the walks/hikes got us away from the crowds. YMMV.

First off, take the earliest ferry you can. Capri gets crowded, but it takes a few hours for that to build up steam. Skip Capri Town, which is basically all glitzy tourist nonsense and is every tourists first stop. Take the funicular+bus up to Anacapri. It’s lovely up there, and the crowds don’t really arrive until early afternoon. For at least a few hours, Anacapri feels like any small town in Italy, with locals walking their kids to school and folks chatting with their neighbors. Taking the chairlift up to Monte Solaro is my only “must-do” and is worth going to Capri for on its own. The view is incomparable.

The one hour “Laser” boat tours around the island are fun, and all things considered I thought it was a decent value. The Blue Grotto was closed due to (slightly) rough seas, but I didn’t mind. No matter what you decide on that front, it being closed is always a possibility. FWIW, after years browsing this forum I can’t recall anyone being a hardcore Blue Grotto advocate. Even Rick himself seems to take the stance that the tourist spectacle around it is as much a part of the experience as the grotto itself.

I think a guide is unnecessary.

It cannot be overstated how pedestrianized is much of Capri, and how much walking (3-5 miles) you could do on a one-day visit, with a lot of sun and heat exposure. We did go to Villa Jovis, but it was a long hot walk, and back. We skipped the Blue Grotto, but in fairness, we had been to a similar product in Croatia. It seems to be a famous time-sucker, often cancelled for high wind or waves.

I'm puzzled by the fascination with boats circling the (lovely) sheer cliffs that make up the island. I'd rather spend my time walking the island. (There is a major public beach near the main marina, but very few, long hike to, other beaches.)

I have only ferried from Sorrento. I think Positano has only smaller boats, so I would take a seasick pill myself, don't know about you. Our ferries carried hundreds of passengers from Sorrento-Capri and back. Return ferry had trucks and cars too. Arrival and Departure slips at Capri were 1/4 mile apart!! (Car ramp needed, of course, for the second boat.) Look for posted signs about that, I guess.

We did the Blue Grotto 40 years ago and loved it. We got on a boat that took us to the area then got on a small boat.

To add, you've gotten answers to a similar question about Capri in a previous post:

https://community.ricksteves.com/travel-forum/italy/capri-adc8109c-3e50-40e0-9010-ffd61039ce10

Yes, the island can be hot but it can also be just comfortably warm or even wet and windy. What it'll be the day of your trip is a roll of the dice! :O)