It makes sense to me that while operating on the water, a pilot should know how to avoid collision with other water users, how to interpret channel markers and the like.
Any seaplane endorsement should cover this. The problem is that it all takes more time than most candidates can afford. It is easier to simply request that a potential candidate for a seaplane endorsement hold a boat license and know this stuff before they start the seaplane endorsement.
As best I am aware, the current MAST requirement for seaplane operations in TAS is a power boat license. This license is relatively easy to get.