In Gen I, Hyper Beam is worth it in most cases with a Physical attacker, even some special attackers. However, now Hyper Beam is not worth it, seeing how you still miss a turn even if you KO the foe. The only Pokemon who you should think about using Hyper Beam on is Porygon-Z.
While that power might be tempting, remember that you can't do anything the next turn, including switching out. As said many times before, this leaves Hyper Beam with a power that is technically at 75. You see it on a ton of Pokemon that the Elite 4 use because they don't usually have to care about switching out their Pokemon or any of that kind of stuff, and just go for the kill.
In Gen I however, Hyper Beam didn't make you have to recharge for a turn if you managed to KO the foe. That combined with a high power made it good if you knew you could KO the foe. However, this was changed in Gen II, making it not worth using anymore.
However, Porygon-Z can use it. With a high special attack stat and Adapdability, Hyper Beam with Porygon-Z is famous for being able to OHKO Blissey. This is especially powerful in the One vs. One metagame, and is good for finishing off a Pokemon besides that. Think of it as Explosion with less risk.
Horsea is actually unable to learn Hyper Beam, just like most Pokemon that haven't fully evolved. You might be thinking of Seadra. It must have been at a fairly low level compared to the opponent, or maybe you just used on a Pokemon of resistant type or high Special Defense.
Hyper Beam is very popular with people new to the game, but it just is not worth it except in the cases I mentioned above.
