Thanks for playin - especially for an hour, the arcade style here will definitely be hit or miss.
Grey means sharks are passing on those tiles, red means you have captured a shark on that line and an angry, faster shark is replacing the previous. They also cannot be caught with a bottle, but you can still use a bottle to "tank" a hit from them in a pinch.
Each shark captured offers passive income daily to try make it more enticing to do.
The progression is mostly just an endurance, stacking upgrades from the store and gaining currency faster via the town hall edicts. Day 21 is when the difficulty hits its peak.