Hi @AquamanProspect, welcome to the forum! 
Looks like a fun project, that you’ve made quite a bit of progress on
That’s definitely not the expected behaviour. The T100 is from well before I started, and info on it is a bit sparse (which I’ve asked about internally), but even in the initial kickstarter page it had a rated max current of 11.5A, and max power of 130W. Your relay says it can support 30A throughput, so assuming all of that goes to the motors you should be able to get ~7.5A per motor.
On the thrust side of things, low voltage means low thrust. Using our Voltage Drop Calculator for the full 30A through a single 16AWG cable yields only a 0.54V drop, so that’s unlikely to be the issue. I haven’t used automotive relays before, but I searched for a 12V 30A one and found this one, which has transition times (operate and release times) on the order of 1.5-3ms, which I’d guess could be too slow to handle the power fluctuations for the ESCs (which are capable of changing the motor’s speed every 2.5ms, and are activating and deactivating the coils much faster than that).
It’s also possible this is causing issues. I’m not sure what firmware settings have been used for your ESC, but our BasicESC comes pre-flashed with bidirectional control, and “low RPM power protection” and “demag compensation” turned off to properly run the low kV motors we use. Our firmware settings are available on that product page, but they may not be directly usable for your ESCs since they have a lower current rating (20A instead of our 30A).
As with all projects, I’d suggest you start with a minimal setup and add components in as you confirm the basic functionality. Try starting with just the battery connected to one ESC + motor and the buck converter powering the arduino, and test the motor speed and current draw. If the thrust is decent there then your ESCs are perhaps not the problem, but to confirm that you can then try with more thrusters at once. Once you’ve confirmed that you’ve got thrusters and ESCs that seem to be working properly, then you can start adding other switches and relays and whatnot into the power path and seeing whether they adversely affect your performance
On something of a side note, you may be interested in this performance expectations breakdown I did a little while ago on using T200s for dive propulsion: