Better Upgrades
It doesn't take years to get enough money to get one upgrade nor seconds, wich is good, the average amount of money earned per wave was good, but still I lost motivation to continue after the upgrades ended.
You should make more and more upgrades with a bigger maximum level (instead of 5, 10) and something like a ultra hidden upgrade that is really expensive that destroys all ships in the screen or makes the submarine immune for some time.
Remember: Instead of making the waves worth less money, make more upgrade levels.