In short, gorebyss could be fun for ultra, but Blastoise has ultra and azumaril has great league locked down pretty tight on water side.
First off, bubble and then water gun is the best fast move for water types if they have it.
Gorebyss, as you said is a bit glass cannon-ish, but it cant deal with Giratina's pure bulk. Suicune with ice beam is better suited, but Gorebyss wants water gun, less energy wasted in overcharge, but DK likely wont hurt giratina too much.
Lumineon, could be fun if the stats go up, but other wise, another pretty fish, water gun's better
Kingler, X scissor could be good agaisnt meganium, it also has the very strong move bubble. Though it doesnt have anything for Altaria/Flygon matches, which is why azumaril tends to take great league as the water type.
Huntale, not sure, as It suffers similar problems to kingler, competition from azumaril, outside of twilight cup, I haven't heard much about quilfish (at this time, I'm assuming due to the limitations in types of the cup's meta not being conducive for azumaril's typical rampaging)
All of them have some meta relevance, but seemingly not more than other pokemon, they have one good move, but their second moves are where they loose out. They seem like they can be serviceable teammembers and can work, but may take more than current waterchamps Azumarill and blastoise
