Despite having a very similar gameplay to Metal Slug, Aqua Ippan is more focused on movement, and has its protagonists rely more on allies to give them a helping hand on their journey back to their school.

Since Itou and Yena were only sent to Bernaco Lakeside to find an artifact underwater, they're ill-equipped to fend off countless waves of Vigilanti troops: only having spearguns and a very limited amount of oxytanks at their disposal.
However, they make up for it with their flexibility and speed, being able to fire their spearguns at multiple different directions and swiftly move and jump around faster than their opponents.

While fleeing from the region, Itou and Yena can encounter captured taggers part of the local 'Spray Gang' that were caught vandalising the army's material. Once rescued, they'll thank the divers by closely following behind and keeping an eye out for anyone trying to sneak up on their saviors!
They can both stun enemies with a cloud of spray and throw their spray cans at the soldiers and their mechas. They can also collect items left around for the diver that rescued them and free other captured kids.

Although one may think these carbonated drinks were left by the Spray Gang, they were actually brought in by the Vigilanti soldiers for their off-time. It is said that just one sip of this reinvigorates and boosts the energy of the drinker.
While the duo of divers can't sense these effects, they do feel like they've gained a second shot at life in case anything happens.

At one point in time, the Palco lake was a popular hub for divers and surfers to come visit. Though after the deluge, the tourists were forced to quickly evacuate the area and leave their gear behind.
Among these forgotten possessions are sturdy oxygen tanks, which Itou and Yena can find and use as weapons by throwing them against their pursuers.

When the Vigilanti began relocating their equipment, they unknowingly stepped on the 'turf' of the Spray Gang. Angered by this, the taggers began stealing and secretly stockpiling these crates around to later use against the soldiers when the time was right.
Maybe if the student divers can save enough of these kids, they'll be able to get access to one of those?