Granted a week late, but
Ebay (or your favourite used kit vending establishment)
Get a Garmin Etrex / 12 / II / III / IV / V / 72 for under £50. Likely get a 12 for around £20 these days. All Waterproof (submersible), all throw out NMEA0183, and gives you a breadcrumb trail to get home with. If you have power on the boat, feed them through the plug and then you have the full 24-ish hrs of the AA backup battery in the handheld as well.
Just be careful of the Etrexes - the very early versions didn't output data, but they are so old they rarely appear.
If done right the conneting cable will likely cost as much as you paid for the used handheld!
As you alreeady have the antenna now, can use these as backup nav as well.....