Good stuff.
Make your baffles cylindrical, so they fit inside the tank with just a very small gap between the edge of the baffle disc and the cylinder wall. If you want a cheap (free) material that's easy to cut, have a look for an abandoned estate agents 'for sale' sign. These are made from a corrugated plastic, which is strong but light, waterproof and easy to cut to size with a knife. I would put three baffles in that tank, as it's quite long.
I'm a little concerned about the ballast tank endcaps (coffee jar lids?). Pressures will reach 50-60psi (4 bar) on a sealed system, so it needs to be strong as you will have several hundred pounds acting against those walls. I'd recommend using some plastic sheet at least 3/8" (8mm) thick tied together with two or three metal rods, and securely bonded into the cylinder at each end. If you get a chance to look at a sheerline or Eden boat, you'll see the tank is of very sturdy construction, and it's not done for fun. Copy it.
It looks like you have a direct drive 500 size motor for power. I see it's MFA, and from memory they do about 1000RPM per volt. At 12 volts this will be far too high revving for the size and pitch of prop you have on this boat. You can gear it down 4 or 5:1, or use a lower revving motor. The former is going to require you to move the servos back a bit to allow for the gearbox, the latter may be tricky to source. I would think you'll want a shaft RPM of between 2500-3000 RPM and motors with those specs tend to be indistrial quality (Buhler, Maxon etc.) and take a little hunting down. Brushless motors are nice, but not really necessary plus they'll push the price up considerably.
I have a feeling the waterpump you are using will corrode- designed for pumping fuel, not water.