The common sense rule of thumb is, if it’s “seasonal,” like a swim raft, you don’t need a permit because it’s only temporary. If it’s “permanent,” like a seawall, you do a permit. Boatlift Mats, for example, are seasonal support for boat lifts, they can go in and out with your lift — they’re seasonal.
Statutes vary by state or province, even by county and sometimes by lake. But typically, “seasonal” temporary use is permitted.