Dont have personal experience about Bangkok seaport, but I´ve been warned several times not to ship anything there, because of possible difficulties.
BKK airport, on the other hand, was pretty straightforward, took about 2 hours to clear the bike. They make a temporary import permit, that allows you keep the bike in Thailand for 30 days.
I would think the procedure should be roughly the same, no matter whether the vehicle enters via airport or seaport, so don´t really have an idea, what´s up with BKK seaport. Without a carnet, your choices in that region are limited.