I suggest being able to craft a telescope that you could equip while you are sailing. If there is a whale within some range (maybe a big square 9x the viewable area?), it would pop up with a "Thar she blows!" message similar to sensing ore when mining.
Reason: finding a whale is tedium! I've heard that some people have bots to find whales but I don't really get how that works. I've just been sailing around, turning the world's supply of tar sticks into peculiar floatsam and haven't spotted a sperm whale yet.