If you have 3 Mains and 9 Subs of Swim Speed Up, you will swim at 125% of your normal speed (1.25x faster than normal). Ability boosts have diminishing returns, so while one Main will get you to ~107%, a second will get you to ~112%, and so on.
Holding the Rainmaker (or being in Kraken form) brings whatever your speed is and multiplies it by 0.8, lowering you to 80% of what your current speed is. That person with nothing but Swim Speed Up on everything would go (1.25 * 0.8), which, now that I do the math, is 1.
In the time I typed this, I learned that there is no situation where the person holding the Rainmaker swims faster than someone with no gear modifiers.