If you're lucky it can be worth it. I mean it's only one click.. so I don't know if it's too much effort. Plus if you collect avvies it's a must have avvie. But you can only get the avvie past fishing skill 85 where you can catch the giant squid thing.
Mysterious Swirly Potion - randomly changes your pet in to any limited pet Flask of Rainbow Fountain Water - randomly changes your pets colour These are the 2 items worth over 1m, need level 240+ or so (IIRC) to fish them out, and some luck!
And how long does it take to get to that level? Over a year? From where I'm standing, it seems much easier to create numerous shells to AB on than it is to fish for a year for a chance to get an item worth 1m+
The best thing i ever got from the under water fishing is a bag of broken neopoints...sold it instead of fixing them because my friend told me i could get more if i just sold them instead of repairing them.
If you've got a daily doer though, I really can't see how it would hurt, besides the junk, but that isn't too bad considering the eventual rewards.