Here is a unit that was designed by Albright and made by Albright for bow thrusters.
Things that could cause the contacts to weld are:
Weak batteries that are not holding up under load.
Bad switch that is lowering the voltage running to the coil of the contactor.
Motor brush that is failing so it is pulling to much current.
The blades not being clean and causing more load.
A motor that is failing which can cause too much load.
Small battery or motor cables that cause voltage drops.
I looked at the reference and noticed they offer suppression diodes . THose would be a good item to order and install. DC motors will create a very high voltage and powerfull REVERSE voltage backflash when de-energized
or shut off.
THere is a lot of magnetic energy stored in the motor coils when operating and when shut down that energy has to go somewhere. Without the diodes that energy will jump and burn the contacts.
THe diodes allow the reverse voltage to run back through the motor windings to be dissipated as heat. THat backflash is of very short duration but if not dealt with will do damage to the control relay and can severely shorten the relay life.as you have seen.
Just a last note is that backflash can also damage electronic equipment if it gets into your general system. Long cables and Batteries will act to dissipate that energy also but better to dissipate it at the source.
When installing just be sure the installed polarity is correct. It must be installed in reverse polarity so the normal operating voltage is blocked.
There should be instructions with them for installation.