Re: ... just replaced my old etrex with a garmin 76map, the reason was, that the etrex series don't show the vmg. ...

The eTrex Mariner (West Marine Exclusive?) adds VMG and a limited marine database. $159.99

I have not used it so can not comment (but its on my Christmas list)

Per the West Marine web site:
"Garmin has produced an eTrex that is perfect for West Marine customers and available only from us! The Mariner has the marine Aids to Navigation database built-in. Accurate and easy to use with virtually every buoy, lighthouse, and nav aid in the US. "
West Marine eTrex Mariner web page