I`ve been there,on commercial boats. There are sightseeing ferries,and an 80ft steel ketch which did trips and overnights. Cruise ships made the Sounds an essential part of cruising NZ.
It is utterly beautiful. Rivals fjords anywhere.Wild and remote. Even more remote is Doubtful Sound where I overnighted on a small(?10pax) commercial cruiser, very remote, just getting there was difficult involving a long road trip and a mountain pass, but well worth the "Lord of the Rings" scenery. We went kayaking too. Bring lots of insect repellent, there are great clouds of voracious sandflies.
You get there from the Tasman Sea which can be rough. Once inside it`s fairly protected by the steep sides but I imagine you could get winds in there and consequent wave action. I think the Sounds interconnect, it looked a bit like The Inside Passage, last time (of 3X) I saw it from deck 10 of a cruise ship.
Highly recommended. NZ alpine scenery is the equal of anywhere I`ve been.
I don`t think you would find much in the way of boating services, you`d need to be quite self sufficient, Milford is probably the major town, basically a tourism establishment, reached by road by passing over and along a mountaintop glacier, with a fast downhill to Milford. I have a good story about catching a tourist coach while going downhill quite fast on a bicycle....fortunately the coach picked up speed..
You`d have trouble getting there at present, I expect you`d have to quarantine for 14 days, Australians and NZers have a "no quarantine" "bubble" arrangement. Very little if any Covid in NZ,they want to keep it that way.