Not sure what seawater has to do with this and more concerned with age degradation although option to upgrade to urethane nice to know. That will also degrade with time but much longer lasting my understanding. Used to be into racing cars and where I recall seeing issues either parts selected as to durability. Same with wanting to build off-road trucks.I asked that question to Foot Masters and there response was :
if you think it's just small amounts (of seawater), it would not be a concern but you could upgrade to urethane if you wanna make sure
Just seems safer not to place degradable components under such a heavy load where later on might be problematic to fix. Especially with my own experience and those racks aren’t as heavy as holding up a fish tank but they were heavy enough that over time I’m guessing the wheels hardened and started cracking under pressure. Obviously not all rubber made the same but understanding being at some point they all degrade with certain factors possibly accelerating that process.