For harbor fishing, you get more points (and are more time efficient) when you catch multiple fish in one cast, so try to be strategic about where the fish are when you bring your hook up so you can get a bunch. Also, always have your hook in the water!
With sardine smash, that can be a little glitchy and the fish will turn red too quickly and won't turn back to blue. If you're having that issue, right click and adjust the quality to low and mute the game. That'll make it run faster. And the fish recovers quicker when it's not all the way busted, so don't stop just before then. On the higher levels, don't waste your fish life on the things that give you the least amount of points if high point things are on screen. Prioritize, basically.