There was a thread on the Practical Boat Owner forum
recently on similar topic. One user decided to take a chance on a cheap
chinese ebay serial
to wifi converter and overcame the slightly iffy manual to achieve the desired result. The cost (< EUR 60) is comparable to or less than the homebrew raspberry pi based solution, without some of the hassle (but also without some of the flexibility).
Obviously such a solution also requires an AIS receiver with serial
output, but there seems to be more choice than with wifi-enabled units (some less than EUR 150).
If going for super-cheap AIS sources, maybe take a look at software
defined radios. Search for "ais sdr". Plugged into a small computer with a wifi dongle capable of master mode you could probably build yourself a decent wireless ais system with networking capabilities far superior to those of some of the commercial
systems which occasionally look as though the network programming has been done by donkeys.
I haven't personally tried the SDR route
as, reportedly, the signal processing software
is a bit of a challenge for my little pi.