We hooked up a microphone to our tower speakers, quite easy actually.
What you need is a mixer.
Here is a suitable one on Ebay:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=23785&item=7330644 464&rd=1&ssPageName=WDVW What you do is put the mixer between the stereo deck and the tower speaker amplifier. Now you can "mix" in the microphone. Note that the mixer needs to be powered up whenever you are using either the stereo or the microphone so you may want to provide a switch to bypass the mixer when it isn't needed.
Also note that it gets really, really annoying to have a microphone when your boat crew starts messing around with it......