If you're looking for manga similar to Hyperdimension Neptunia: Nep no Natsuyasumi, you might like these titles.
The world is in a state of isolation. The cause is due to mysterious deep sea entities the navy has dubbed the "Deep Sea Fleet" and they're taking over the seas, isolating all the countries of the world from one another. Refusing to just sit idly by and let the Deep Sea Fleet have its way, all the countries of the world pool their resources to make a new weapon to combat them... a weapon that is comparable in power to that of a battleship... and its name is: The Fleet Girls. Will this new weapon be enough to combat the Deep Sea Fleet? Will the ensign that was just sent to the naval base where the "Fleet Girls" are kept be able to aid in their struggle... Only time will tell as this war against the Deep Sea Fleet begins to unfold.
Having becoming one of the “shipgirls” possessed with the powers and memories of the War of the Past, Zuikaku and her elder sister Shoukaku reunite, and through the man known as “The Admiral” comes to terms with the existence of humanity’s enemy–the Abyssal Ships. She resolves to protect this world that is so similar to that of her memories and embarks once more onto the battlefield–and must experience and confront the implications of her own cruel fate.
Young Wataru Mitani's life is a mess. His father has abandoned him, and his mother has been hospitalized after a suicide attempt. Desperately he searches for some way to change his life--a way to alter his fate. To achieve his goal, he must navigate the magical world of Vision, a land filled with creatures both fierce and friendly. And to complicate matters, he must outwit a merciless rival from the real world. Wataru's ultimate destination is the Tower of Destiny where a goddess of fate awaits. Only when he has finished his journey and collected five elusive gemstones will he possess the Demon's Bane--the key that will unlock the future. Charity, bravery, faith, grace and the power of darkness and light: these are the provinces of each gemstone. Brought together, they have the immeasurable power to bring Wataru's family back together again.