I do alot of offshore fishing and have exp with GPS's/plotters.
The Garmin 526 is soley a GPS/plotter unit. It displays picture of nautical charts with hazards, navigation aids and depth but its not a depth finder. Companys make units that offer split screen GPS/depth recorders but this isnt one of them. He gave you a manufacturer name and model number so I assue he knows what hes talking about but its odd the way you describe he wants to know the depth. Thats usually not what a GPS is used for. A depth finder itself is much more accurate and useful for finding fish. I would verify with him what exactly he needs. And as mentioned already, there are cheaper alternatives to GPS and depth finders that work well. The Garmin is a good unit but in the marine electronics game, you get what you pay for. You'll get more informative charts, color opposed to B&W, better resolution and navigation ability.
Interesting that cousin would ask for such an expensive gift. My cousins and I will buy a couple beers for birthdays and call it a day.