Do we always have to go to the rental booth to rent a boat? Or can we keep it and use it anytime after we rented it once?
you have to go to the booth :’(
I’m just watching a stream from Erose and she went with the boat to the unknown island, disembarked and explored the island and I wondered how she would get back, but the boat remained at the shore where she left it and she could embark again. At least this is possible 
This one is simple.
Once you rent a boat DO NOT return it to the disembark area (end of pier).
Instead, Park it on the beach. It will remain there until you either return it or replace it.
No need to rent the boat every time!
PS; Don’t forget where you parked it. The boats do not spawn until your right next to them. Will save you some time searching for it.
Thank you for this information, I have not got as far as renting boats yet. I look forward to a time when - maybe just maybe - we can own boats, and possibly even live on them. Probably just a pipe dream, but that would be so cool.