We've recently started mutually exchanging AIS data with MarineTraffic.com. aprs.fi gets AIS data from MarineTraffic, and MarineTraffic gets what we have. This improves the coverage of both services significantly, as there wasn't much overlap before.
The peering is still being tested and the protocol improved, so it might be a bit flaky for a while, but generally it seems to work.
We're not converting AIS data to APRS format, and it is not my intention to do so - too much valuable information would be lost in the conversion process, and the APRS-IS network simply is not there for this purpose.
We've specified a new JSON based protocol which is also useful for feeding AIS data from receiving stations to these services. There is already a working Perl implementation for converting an NMEA AIS stream to the JSON AIS protocol for uplinking to aprs.fi, it's currently being tested on a couple of receiving sites. I'm hoping the author of the client will release it in the near future.