Designing a DC 12V 80A Power Distribution PCB panel and heat sinking
Design Considerations: Cable power loss %, Battery energy density (Wh/kg), Buoyancy ( air presure force vs battery weight), wall pressure, choice of motors, battery and controller.

At least learn Ohm's Law V=IR and \$Pd=I^2R=V^2/R=V*I\$


Get a DMM  (cheap) and measure Motor winding Resistance or DCR or get locked rotor=surge current spec.


Cable Loss
You can look up AWG tables to get Ampacity @60'C or compute loss, Pd from above using \$\Omega/m\$ vs cable cross-section area or gauge or measure flat braid 1cm*1mm wide for example. There will be voltage drop. You want your cable loss to be &lt;2% of surge current to reduce loss of torque. 


motor  torque = current   ( prop load and acceleration of rotor mass)
(no load) motor speed = Voltage (V/RPM)
Motor power rating = rated current*V
Surge current = 8 to 10x rated current.


e.g. 
worst case surge from I=V/R (for Vbat and DCR of motor coil) is up to 8x to 10x8A or 64A to 80A per motor !! So start up slowly
if you already have motors picked out OK, although I would use two >=21V drill motors, battery controller and all modified to work inside the sub with quadruple "O-ring seals" around the prop shaft made for high pressure.
Best wiring is multiple strands of insulated magnet wire called Litz Wire due to low inductance, which can cause EMI interference with electronics unless shielded and ferrite filtered in pairs.
next best is heavy braid like car battery cables. 
Then use Terminal blocks for distribution like fuse panels in cars
Traces on PCB may need solid copper AWG 16 with low R = RdsOn to get same low loss as per above equations.



Buoyancy


\$F_B = ρ_f V_f g \$
\$F_B\$ is the buoyant force, \$ρ_f\$ is the density of the displaced fluid, \$V_f\$ is the volume of the displaced fluid, and \$g\$ is the acceleration due to gravity, \$9.8 m/s^2\$. 
It's very important to remember that the density and volume in this equation refer to the displaced fluid, NOT the object submerged in it. 


Consider Water pressure is 1Atm per 10m depth so 100m depth is 10 atmospheres or 150 psi, ( I recall) and see if SLA will float or sink vs LiPo. Then make structure like a tank to survive this pressure and the best seals to match...