''Umm..You know that in real life male seahorses give birth so I was thinking if it works the same in Pokemon.''
In Pokémon is very different. So to answer your question, both gender can work for breed a Horsea.
Male or Female with a Ditto or Male and Female parents can breed a Horsea.
Both parents and Horsea have a 50% chance to be male or female.
In Bulbapedia's Horsea page, talks about that males raise the eggs more often than the females.
Hope this helps.