What type of polyurethane?  Your best bet might be to find a large polyurethane manufacturer, and call their tech rep.  Explain your problem, ask for their best product and tech sheets.

The navy uses a lot of Adiprene L100 and other types for naval applications, under water....... for what that's worth.

good luck!

linda01,
check on the plastic forum or linked your question there..
a lot of plastic crack issue there.check it out.

regards

There are two types of polyurethane. The first type is  thermoplastic for injection molding and extrusion, etc.  The second type is thermosetting, for adhesive and sealant applications. Curing, at room temperature or elevated temperature, is required for thermosetting resisns. As mentioned by Orangepeel, working with the supplier to ensure if the grade used has adequate mechanical porperties/chemical resistance (including water) is very important. If the thermoseting type is used, I would also verify if the curing condition used was adequate, as the phenomenon described could be related to cohesion failure. If water immersion is imporatnt, make sure "polyester" type polyurethance is not used because polyester component is susceptible to hydrolysis.