Never bring a knife to a dart fight.

Heckler & Koch P11

The HK P11 is a Heckler & Koch pistol designed as an underwater firearm that was developed in 1976. It has five barrels and each fires a 7.62 X 36mm dart electrically. Loading is by means of a five-round case. The design resembles that of a pepper-box firearm.

SPP-1 underwater pistol

The SPP-1 underwater pistol was made in the Soviet Union for use underwater by Soviet frogmen as an underwater firearm. It was developed in the late 1960s and accepted for use in 1971. Underwater, ordinary-shaped bullets are inaccurate and very short-range. As a result, this pistol fires a round-based 4.5 millimeters (0.18 in) caliber steel dart about 115 millimeters (4.5 in) long, weighing 12.8 grams (0.45 oz), which has longer range and more penetrating power than speargun spears. The complete cartridge is 145 millimeters (5.7 in) long and weighs 17.5 grams (0.62 oz).

Follow the links in the guns' names for more info.

These weapons are from the cold war era. They had a rather short range (not effective over 20 meters underwater or 30 on air). With a bit more tech advancement, and the demand caused by your species living underwater, and your world could have versions with longer range.

Also, torpedoes to do the job of our world rocket and grenade launchers.

And trained war sharks. Just because sharks.