Just came here to say that actually, wahahaha.
Fighting the lagiacrus underwater in the flooded forest is just a horrible mess of a thing, any area that he's underwater he takes up most of the lane and he's constantly electrocuted so you get thunderblight and ahrgh agrhhrg hurggh
Just gonna say one thing: LANCE AND GUNLANCE. The gunlance has FIRE ELEMENT on its shots. Lances are some of the easiest and safest way to take on the underwater monsters. It's even worse when you get a fire element lance because almost all water monsters are weak to fire in some way or another. At least, the main ones are. Ludroth, Gobul, and Lagiacrus are all weak to fire.
Although, I think the Gobul is kinda weaker to thunder, but eh, he still has a minor weakness to fire.
The only problem is tailcutting because you're so immobile with the lance/gunlance. Also consider investing in Barroth armor with the lance because it gives Guard +1, which is so so very good for weapons that are dependent on blocking.
Also, holy shit, I just discovered the Volvidon gives Evasion AND Attack, which is just frankly incredible, because I'm a huge fan of Evasion. In fact, a lot of the higher armor sets have really good stats, like far better than the ones they had in Tri. Gigginox armor is way better with Earplugs and oh, nevermind, that's not Sharpness +1, that's speed sharpening. Gigginox armor is probably still bad in most cases. Actually, with Reload Spd/Status, it might be good for bowgun, but that's about all, I think. It'd be for specializing in status, but that's all. Which isn't bad, I guess.