This phone from Samsung has a 3,000 mAh battery. A water- and dust-proof casing with an IP68 rating means a quick dip in the ocean is no big deal.
From Back Market
Galaxy S9+ 64GB - Black - Unlocked
Gadcet
Samsung Galaxy S9+ 64GB, Lilac purple - Unlocked, New
OnBuy
(Midnight Black) Samsung Galaxy S9+ Dual Sim | 64GB | 6GB RAM