It might be effective to help locate the unit if it drops overboard or for your own boat to recover you in a MOB situation, but it definitely would lack the brightness needed to attract attention from a distance. Also, it's probably directional to a large extent.

The model I got is the HX850, the predecessor to the HX851. It has the GPS and you can display position/speed/COG, mark way-points, and the GPS is integrated with DSC. But it doesn't have the strobe or some of the more advanced GPS "navigation" functions such as routes that the newer HX851s has. I got it on close-out for about $150 whereas the newer model was about $100 more at the time. IMHO, the strobe is a "nice to have" but I pin a better light inside my inflatable PFD anyway, and the GPS route functions aren't important to me (I use more user-friendly devices for following routes).