Underwater Province Defense?

[JimMorrison]

All I am saying is that the logistics of invasion are vastly different if you cannot do PD at all.

You can perform other, specialized forms of deterrence, but with their high cost, and inability to ever move more than 1 province/turn, they can't provide the same effectiveness as small amounts of PD, as far as forcing someone to commit larger forces to avoid attrition, or to at least commit something of value that they will fear losing.