The weather thing is true, at least for me it is, when fishing clear rivers, fish are more likely to go deep on sunny weather to avoid the direct sunlight that isn't good for their scales. As far as the ocean is concerned, I don't think it really makes much of a difference, if anything, it would improve it slightly, fishign at deeper than 150 ft, there is very little sunlight anyway, and the slight raise in light wouldn't be enough to harm the fish, bu would give lures a bit more flash and make them easier to see. Can anyone else attest to this ?
I'm not so sure about rivers, but in teh ocean, the meeting of different "types" of water can churn up bait fish, and the bottom, so fish come in to feed right along this "tide-line," can be very productive sometimes.
Fish may jump for a variety of reasons, yes, I have seen them jump to avoid predators (seem a salmon jump clear of the water, to be followed quickly by an orca or seal. Another "theory" I heard from somewhere/one was that they are tryign to "see" where they are going in the rivers, I don't know how much truth there is to this, but in some places like the Fraser, with a visability of 8 inches, it may be slightly more believeable. My last idea as to why fish jump could be related to parisitesand "itches" that can sometimes be lost when the fish slaps back down on teh water's surface.
These are just a few of "my" reasons, but I'm sure there are a whole other set of theories as to why do the things they do.