Most of those right apply to new goods -- including a new boat! -- where the specification is clear; if it doesn't do what is claimed by the seller, you're entitled to a full refund.
It doesn't apply in the same way to secondhand goods like boats sold through brokers which will inevitably not be "perfect", and may well have issues that neither seller or broker is aware of.
A deposit on a boat is a way to form an initial contract between a buyer and seller, in exchange for which the seller may do things like taking the boat off the market. If significant unexpected faults are found then the deposit is usually refundable, if not -- for example, the buyer just doesn't like it, or changes their mind -- then part or all of the deposit can be kept by the broker, so long as this is made clear in their T&Cs.
This stops unscrupulous buyers with plenty of money putting deposits down on a whole raft of boats they fancy the look of to get them pulled off the market, then asking for all of them to be refunded, then repeating -- which would put less deep-pocketed or ethical buyers at a severe disadvantage, as well as screwing the brokers.
There's always a balance to be made between the rights of buyers and sellers, and deposits are a way to try and balance out both. If you don't like the terms of a particular broker, don't use them... 😉