How to slow down a seaplane on water?
Unless you're planning on trucking a seaplane out of where you have landed it, you usually don't need to worry about slowing down on landing since you can land in a far shorter distance than you can take off. In other words, the landing water run is far shorter than the takeoff water run.

If you do want to shorten the water run or reduce your taxi speed, holding up elevator will do that. A few seconds after touching the water and at idle power, you can use full up elevator without fear of the aircraft lifting off again, and it will add the drag of the elevator plus change the attitude of the floats or hull to a high-drag, plowing attitude. I used to do that if where I wanted to dock was aft of the touchdown point and I wanted to quickly slow to a safe turning speed.

Most constant speed props (variable pitch) on reciprocating engines don't have a reversible pitch or beta, but that can be purchased as a modification on some. The major use of such is as an aid in docking, and for that it's really useful.

Steering left and right repeatedly doesn't impress me as a very good idea, but I suppose it would help a little, but very little since it wouldn't be advisable to do it until you're well off the step.

Deploying the water rudder adds a little additional drag. Depending on the wind conditions, putting in a little rudder to match cross-controlled ailerons also adds drag, but that's not generally used to slow down as I remember. It is used, though, when sailing the aircraft, for which it's standard procedure.

See also Chapter 4 of the FAA Seaplane Handbook.