The new 3085 sealed tanks are not able to be deployed on or later move into water.
Also Environmental Sealing as such is asking for Fusion Engines, but that is meant for Vaccum, not water units. (conflict with a single unit though: Striker tank, perhaps an errata for Catalyst?)
Fuel Cell engines should enable Environmental Sealing, as of now only Fusion is being legally accepted in megamek.
As for the other parts of the comment, ignore it... seems like errata for Catalyst as there is no reason for SRTs if they can't go under water.
The code does not check for fusion engines and environmental sealings.
Well... it did show an "F" on the Add a Unit table by the Scorpion of 3085 mentioning "Fusion Engine is necessary for Environmental Sealing". I'm speaking about the current build.
And in fact Fuel Cell should allow for it anyway.
Please attach the file in question.