Breloom is a very useful Pokemon in Emerald, just as a bulk up/sky uppercut user and not a grass attack user. All grass attacks are special and Breloom's special attack is pretty low.
The best water counters in this game are arguably calm mind users, because most water Pokemon will just spam their special moves and let you set up however many calm minds you want. I think the best calm mind users are Gardevoir, Kadabra, and Golduck because they have the highest special attack. (Xatu also has good special attack but it has a bunch of common weaknesses)
If you still want a thunderbolt user, some Pokemon you should consider in addition to Manectric are Gardevoir (learns calm mind), Magneton (higher special attack than Manectric), and Starmie (resists water and learns a bunch of other useful coverage moves).
Because Gardevoir learns both calm mind and thunderbolt (and has very high special attack), it's probably the best option for you. Here's a moveset.
Gardevoir @ shell bell
Ability: it doesn't really matter, neither are all that great.
EVs: 100 Spe
- psychic
- calm mind
- thunderbolt
- reflect