With a 400 you _might_ not have to do anything. Slow Sticks are flying at twice that weight in AP use. It's only 3 oz heavier than mine, with a stock 300 motor and 6 cell NiCads.
You _are_ going to have more drag with the floats - that's really the only potential concern. If you have enough thrust to offset the additional drag, it'll fly. If you're used to flying it at 1/2 to 2/3 throttle, you should be fine.
I have dunked several seaplanes.....:-( Assuming you did all this in fresh water, just open up the electronic component cases/sleeves, blow the water out and let dry in the sun. All should work OK once again. Likely the ESC or rcvr got wet and the water was/is the source of your glitches. Try enclosing the elex in plastic bags to semi-protect them. I'm not sure of the effect on cooling the ESC with a baggie over it, but I am pretty sure water spray is a worse problem. A little vasoline on servo cases will keep the water out of them.
If water is getting on the plane, (I'm assuming this is not a seaplane, but a float conversion) there needs to be some spray control done, with some spray rails or deflectors. I never got more than a drop or two on the fuselage, unless I plunged it straight in!
Possibility 2, there was a local source of interference jamming your signal.
You never did say if you were direct drive, size and pitch of prop, gear ratio, if it has one. Also size of batteries, and amp rating of ESC. Would you mind helping out here? I have a motor that is looking for a home, and I have not done electric before. I would like to do a float setup about like yours.
You didn't have the same conditions in the hobby shop that you had when you were flying. One of many problems I've encountered with GWS Esc's is the BEC output. They won't handle much of a load at all. I've found that anything over... two.... servos usually creates a problem , or any other heavy load such as a stalled servo , stiff control surfaces , binding control linkage etc. Most GWS Esc's are rated for 1 amp , but like many companies they're stretching it a bit.
I'm not sure how the ESC is wired , but maybe someone here can enlighten us on this.
I'm pretty sure that any excess load anywhere else in the system such as the motor will rob the BEC of power. Also , I think the BEC power will drop as the batteries discharge.
In the last few weeks I've test flown ( and crashed ) 3 airplanes with GWS receivers and ESC's . One was a GWS C-130 , a GWS A-10 and another Arf of some sort with GWS equipment in it. The C-130 had four servos. Had I known he was using a GWS Esc with four servos I would not have flown it. The servos locked up after once around the pattern and it came in hard and pretty much trashed it.
The A-10 barely had enough power to stay in the air, it started glitching and finally quit responding to control input. I think the motor was pulling too much current and caused the ESC to quit , BEC and all. It smoked the ESC.
The other looked something like a Hobby Lobby Wingo , only bigger. It started glitching bad and I lost it. Not much damage. A Hitec receiver and a Jeti ESC cured his problem.
I'm not saying this is definately your problem , but you did ask what WE think about it. Since I've seen so many problems with GWS ESC's and Receivers , I think it's very likely that is your problem.
Anymore , I always ask what kind of equipment is in the plane before I fly it.