Avoid jellyfish at all costs.
If you bring any jellyfish to the surface shooting them will inflict a penalty against your earnings. Also any of the fish, touching them on the way down will cause your line to rise back up if it hasn't been upgraded.
The dual Uzis offer the most effective assault against your flying fish.
Toaster upgrade is a usefull one. The first fish you touch on the way down will be electrocuted, and you'll be able to carry on downwards until you hit a second fish.
If you've got a chainsaw upgrade attached to your line, holding down on the screen will not only sink the line more quickly, but it will also slice through any fish you come into contact with for a quick injection of cash. Make sure you don't run out of fuel and end up getting your hook caught early on.