Coast Guard Small Boats usually don't show up on AIS--at least not here in Florida. As far as I know, they're all equipped with Blue Force tracking, which is an encrypted AIS. Cutters usually turn on AIS when they're in the vessel traffic system here, but do have the ability to turn it off when they're on LE cases. Also, many small boats use MMSI numbers from SeaTow or TowBoat US which are not available in any public database that I know of. The only public database of MMSI numbers would be the FCC ULS, which was referenced above.
Out of curiosity, are fire/rescue boats using DSC and/or AIS in your area? If so, how broadly have these technologies been deployed? I think it's unlikely that they'd use DSC that often...it's usually more trouble than it's worth, unless the person you're hailing isn't answering and you happen to have their MMSI programmed into the memory of your radio. AIS, on the other hand, would be very useful for SAR and rescue vessels, providing information about the current location of the rescue vessels.
Remember that DSC doesn't transmit the MMSI with every transmission, so you can't scan for a specific MMSI number. It's mainly used for initially hailing another vessel and moving them to a working channel automatically. Other features include position exchange and distress broadcast when connected to a GPS. If you want to monitor DSC, you can download some DSC decoding software and connect a radio with a discriminator tap to your sound card.
Are you trying to find these boats on AIS? If so, rescue and law enforcement have their own categories. You might be able to filter by that to remove clutter from other shipping activity. Is your AIS running locally, or are you using a web service?
