I see more consistent spectral currents when I wait until the last minute to register to board and end up on a boat with only 8-12 players than I do with the full boats. I highly doubt people leaving is negatively impacting the chance for a current to spawn. It might actually be helping if the chance to spawn inversely scales based on how many are on the boat as some suspect.
There are no Alliance bonuses to the best of my knowledge. The bonuses are self and party. Another party leaving the boat is not going to affect your party bonuses. Having 1-2 players drop from the party you're in likely won't affect the 10% party bonuses (the higher bonuses aren't going to get earned without a pre-formed party specifically targeting those bonuses).
You don't need 3 currents to get the mount (though it certainly makes it). I've seen several players getting the 10k+ score with only 2 currents, and came close myself on a couple of those occasions. Learning to catch the fish that require Fisher's Intuition can help offset bad spectral current RNG.
Stop worrying about what others are or aren't doing on the boat. Relax and have fun fishing. It's not designed to guarantee a good haul on every trip. That so many players have already managed to earn the mount is a bit surprising, you'd think it would be more rare with the content just over a week old.
