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 Amazon
Samsung Galaxy S9+ SM-G965F 6.2" Dual SIM 4G 6GB 64GB 3500mAh Gold Galaxy S9+ SM-G965F, 15.8 cm (6.2"), 6 GB, 64 GB, 12 MP, Android 8.0, Gold
OnBuy
(Midnight Black) Samsung Galaxy S9+ Dual Sim | 64GB | 6GB RAM