For the simplicity of this guide I'm playing the game on Easy Mode.

First thing you should know, it doesn't matter too much whereabouts on the map you go to, as long as you don't go to the bonus levels.

My understanding is that the game always plays at the same speed each time. So your diver will reach the bottom of the ocean at the same time if the sharks didn't interfere, the problem is they do.

If you reach the bottom of the ocean but you still have sharks/pirranahs to kill, the end of level bonus won't start until you've killed them all. So the main trick is kill them before you reach the bottom. In some cases it's a good idea to not finish typing out a word because then the shark stays on screen and the next wave won't be able to spawn.

For the boss levels, the same logic applies. Your diver stops moving once you reach 260 feet so you want to time your last kill to just before then to get the most optimal timing. When the boss arrives on screen, get as many hits on it and get them on them as quickly as possible.

Also the shark zapper can actually be useful here, in some cases it can be quicker to just use the zapper rather than typing out the full word. Especially on sharks that take more than 1 hit.