Yes, they finally made a female Robot Master.
The first part of Splash Woman's stage is pretty straightforward. Simply dodge the spike enemies to the best of your ability, they'll explode eventually. The octopus robots can be destroyed by shooting their heads when they pop out. If they hit you with their ink, don't worry. Apart from your character's appearance changing, there are no harmful side effects. And this can easily be undone by going to the subscreen and returning to gameplay.
The first real challenger in the level comes from a "bubble jumping" section. This will look familiar if you played Wave Man's stage in Mega Man 5. You can stand safely on the big bubbles, but stand on the small ones for more than a second and they'll pop and you'll fall. You need to stand on the large ones, and hop across the small ones to reach the other set of large ones - all the while dodging the speeding fish that cruise by the area.
The other big challenge comes from the hydro-powered platforms that come out of the walls. As if these spiked walls weren't enough of a problem, these platforms only have a little bit of water in them. When the water's gone, the platform vanishes. In other words, watch the patterns in these rooms closely. Use Rush to alleviate some of the pain if you can.
Splash Woman herself isn't too hard, if you know her techniques. She'll always start by singing which summons three waves of fish - low, mid-range, and high. Simply dodge or shoot them to avoid taking damage from them. When she reaches the top of the stage, she'll rain flashing trident shots down on you. Dodge these as best you can. When she's done with this, she'll sing again and repeat the pattern for the remainder of the battle. If you started here, hit her with your arm cannon while she's low and singing. If you got the Hornet Chaser, use it while she's raining trident shots (do not use it while the fish are present or it will just chase them!) All in all, she's not too hard to take out, especially with Hornets.
Victory gets you the Laser Trident, an extremely powerful straight-firing weapon that goes through tons of things and can severely damage enemies.