Not all mashups show locations on land. Here are some "at-sea" Google Maps mashups:
Global Ocean News Map - The Ocean Channel has made use of DIY Google Maps creator Mapbuilder.net to create the Global Ocean News Map. The news-map aggregates ocean-themed stories and places the stories on the map where they are taking place with a nifty little "O"-cean icon! :)
World Dive Atlas - WannaDive.net is a site that allows you to post and read about world dive sites. It has begun to integrate Google Maps throughout the site. A Google Map is now displayed on many of the featured dive sites, and the Zones and Countries section has overlays of various dive sites within that region.
Shipwreck Map - This Google Map mashes up with the shipwreck database published by the NOAA. The info-balloons shows you the wreck name, depth, date and latitude and longitude of the shipwreck. Search the database if you're trying to locate a specific wreck.
Submarine Wreck Map - From the same site that has produced the Shipwreck Map, this map helps you locate sub-wrecks within a certain radius. Center the map using the circular target, then search for the nationality of the wreck.
Here some other previously mentioned Google Maps mashups that plot locations over water: