I've never done fishing treasure maps, but for the regular land based versions there is some nice stuff. For every lvl 5 map (devious) and lower you have a chance to get a map 1 level higher (for both trammel and felucca maps). The map is randomly felucca or trammel. Once you get to lvl 6 and above (goes to level 8 ) only the felucca based maps will give a chance at a treasure map upgrade, and again they will be random tram or fel.
In general the loot sucks. The armor/scrolls/weapons are generally trashed, but the reagents can be sold on [exex, and if you have a gem pouch you can keep all the gems as well. If you choose to empty out the chests at the higher levels the monsters can drop a good amount of gold (lvl 6 and above get balrons/ancient wyrms), or an additional treasure map occasionally off one of the monsters. With gems, reagents, and the gold from chest/monsters you can make some great gold, and on the lvl 6 maps you could get a decent artifact. There are some custom loot items that are explained
http://uoex.net/wiki/Treasure_Hunting here, but asides for the lvl 8 ones (very very rare) there isn't a very high demand. The +3 and +5 lockpicking gear you can sell, but cartography is pretty worthless.