Best bet is to deploy a series of expandable sonobuoy, like the ones on MH's and LRMP's, and monitor from a small console on the bridge. We already use one or two such buoys deployed from Rhibs and a monitoring console ashore for anti-combat-divers harbour defence. If you want to be cute, fitting the Orca's with a helicopter dipping sonar over the stern would be an easy mod. For the MCDV's, they are big enough to get a full tail on their stern.
The real problem is ASW weapons on such small crafts: Nowadays our primary weapon is the Mk46 torp and that plus the launch tube would be pretty heavy and cause stability problems.
Question is, really, why would we want to do that? For harbour approaches, much better to deploy fixed bottom ASW sonar nets monitored from shore and then use a helicopter for prosecution.