Electronics never have been my strong point and I'm sure you have already done what I would do (double check all connections for loose or corroded from unit to sensors and all the way to the battery). But anyway, it sounds to me to either between the battery to the unit or internal of the unit since you are having problems with both water temp (which is located on the transducer) and GPS ( GPS puck).
SO I'd check all connections and if found nothing then call Lowrance ........................... after all, if we had your money we'd done bought one of them 24" Color, GPS/Radar, depthfinder/sidefinder, Fuel Consuption/ Engine Monitoring/ everything hooked-up on a 'Backbone' Cable Jobbies !
Good Luck !
-391
p.s. The reason I mentioned calling Lowrance is they might be willing to give you a credit on your X-15 if you send it in and 'up-grade' through them............. just a thought !