Rocks underneath may have smashed it if there was high waves/storms. Learned that the hard way. The boats can really bob up'n'down and get wrecked fast. You may get lucky if you can swim over the resources to retrieve them from the shore bottom if they are visible (shallow water) ... or you may have to build a high-dive platform to get your player to go deep enough to get them. Saw some Youtube Vids show this and it works to a certain depth point.

And as stated above, always carry Portal resources.