recommendation for renting a boat to drive around the lake?
There are only two boat rentals:
https://fuschlsee.salzkammergut.at/en/detail/austria/poi/500317/boat-hire-stadlmann.html
https://fuschlsee.salzkammergut.at/en/detail/austria/poi/500096/boat-hire-edenbergerreindl.html
Don't expect a fast power boat. They are prohibited on most lakes in Austria, let alone that a license is required for both the boat and the driver.