The FAA doesn't have jurisdiction over operations landing rights water.
There are some basic right of way rules in 91.115, but it is the same
as basic boating right of way rules.

The question of whether landing a Seaplane on a lake almost alway comes
down to this. What agency has juridiction over the boating on the lake?
Even though a Seaplane isn't really a boat, the agency that controls
the recreation on the lake is the one that would control the Seaplane
landing rules on the lake. Seaplanes are only subject to Coast Guard
rules if the Coast Guard has jurisdiction over the water way, which is
NOT always the case, it just depends..It can be the county, city, state
or some Federal agency, all depends on who manages the recreation on
the lake.

I usually recommend a pilot contemplating landing on a lake he isn't
sure about, call the agency that administers the boat ramp, checks for
life vests, etc. If you can't find out who that is, visit the local
marina or boat ramp and ask around.

As a Seaplane owner, I tend to think that Seaplanes are no worse than
power boats. They don't cause particuarly big wakes. And their
operations on the lake are fairly infrequent compared to boating
operations. The usual complaint is noise, which is a consideration.
Careful choice of take off and landing will usually limit noise to
reasonable levels.

So far as seeing a kayak goes, an object as large as a kayak is no
problem for a Seaplane to see. Contrary to what most people think,
Seaplanes don't need special "sea lanes" or landing zones on the lake.
They just pick a spot that is open and become like a boat once they
land. Technically Seaplanes usually aren't considered "water vessels"
which usually excempts them from any speed limitations on water
vessels. There is always some question whether a Seaplane can land on a
lake that doesn't expressly permit Seaplane usage, but doesn't have a
rule against it either. This one is best worked out on a case by case
basis, using common sense.