regular guns and ammunition are only lethal at point blank range when underwater. otherwise the projectiles just produce some cavitation bubbles as they slow down and drop to the floor after like 5-10 metres. railgun physics basically. i guess it could use the same status effect for when sodium cathes fire when in water. so i guess the gun can fire two different projectile types when that happens. one for out of water, other for underwater.
special underwater flechette guns and spear rifles are required to effectively slay creatures underwater. these spears, since they don't normally fly straight when not in water are only lethal at very close range. unless it's spinning like when fired from a coil rifle. to defeat armor underwater, the spears would need an insane amount of momentum. so mini shaped charges could work for that.