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 Plus 64 GB (Dual SIM) - Midnight Black - Android 8.0 - International Version
OnBuy
(Midnight Black) Samsung Galaxy S9+ Dual Sim | 64GB | 6GB RAM