[3] She was the first ship of the United States Navy to be named for the tuna, a large, vigorous, spiny-finned fish highly esteemed for sport and food.

Attached to the Atlantic Submarine Flotilla, G-2 spent the next five months conducting dive training and engineering exercises with G-1 in Long Island Sound and Narragansett Bay.

Attached to Division Three, Submarine Flotilla, Atlantic Fleet, the boat joined G-1, tender Fulton and tug Sonoma, for a cruise to Norfolk, Virginia on 25 March.

Ordered back to New York for an overhaul, the submersible again transited the familiar waters of Long Island Sound before arriving at the mouth of the East River on 22 June.

Following those successful evolutions, during which the Trial Board noted numerous items requiring modernization, the boat moved back to New York for an overhaul on 14 January 1916.

Six months later, G-2 shifted to the Lake Torpedo Boat Company yard for completion, receiving new diving rudder gear, hydroplanes, electrical wiring and a new crankshaft.

On 21 August G-2 sailed to Boston, Massachusetts via the Cape Cod Canal to operate with the destroyer Aylwin, submarine chaser SC-6, and steam yacht Margaret.

On 12 September, Thetis experimented with a magnetic detector while G-2 lay on the bottom in 86 ft (26 m) of water and, in November, G-2 even conducted experimental work with patrol seaplanes.