Generally, the first three rules of float flying are: (1) you need a boat. (2), (3), See rule 1

You are right. If the engine quits, you need a boat to go get it.
You don't need a BIG boat. A canoe or a jon-boat is adequate. Even an inflatable will do. But just like whenever you drop a piece of toast it will invariably land jelly-side down: Even on a small pond the plane will drift to an inaccessible part of the shoreline.

The only exceptions are small ponds with brush-free banks.
These would either be stock tanks dug by farmers to provide water for the cattle, or water hazards on golf courses. Golf-courses have their own hazards: Strategically planted trees, and Dilettantes with funny shoes and nasty tempers. Pastures only have murderous 3000# bulls. I can deal with cattle, but golfers are beyond my abilities to reason with. My club's former field was adjacent to a golf course, and there was no end to the whining and crying whenever anybody let his plane get a little over the line.