I see the Repeatit, linked by Sarabande, is a directional antenna. This may be a problem at a mooring or anchor, though would be fine in a marina.

The Radio Labs Wave RV II is omnidirectional and has a USB connector. This is constrained to the usual USB limit of 15’ (though maybe more with a booster?). Personally I had problems with the RV I but I know several people who are very happy with them. Don’t know if there is a UK distributor, but you could probably get it shipped from the US – it doesn’t weigh much. http://www.radiolabs.com/products/wireless/waverv2.php