File:Swim Speed Up Chart.png

Formula
x = 10*MainSlots+3*SubSlots

Speed (DU/f) = [base speed]*(1+(0.99*x-(0.09*x)^2)/120)

Base speeds (DU/f):
 * Normal: 1.92
 * Heavy Weapon, Ninja Squid: 1.73
 * Rainmaker: 1.54
 * Heavy Weapon with Ninja Squid: 1.44
 * Stealth Swimming: 1.34
 * Heavy Weapon with Rainmaker: 1.25
 * Heavy Weapon Stealth Swimming, Heavy Weapon with Ninja Squid and Rainmaker: 1.15

Run speeds for comparison:
 * Running: 0.96
 * Running with Rainmaker: 0.77