It depends on the pokémon, anonymous. Sandshrew and cyndaquil normally have no problems with water, although sandshrew shouldn’t be bathed regularly to maintain its dry skin. Cyndaquil, meanwhile, absolutely can go to the beach, although be sure to remind it that it can’t ignite its flames in the water. It doesn’t exactly hurt cyndaquil to do so, but if it exposes its flame sacs to water, it may struggle to light them later.
On the other hand, you should never submerge pokémon that are primarily made of sand, rock, fire, or lava, and never wet a pokémon with external flames. For example, while a charmander may be able to tolerate a sprinkling of water, it should never be submerged or bathed, as that will extinguish its tail flame. Likewise, if a slugma’s body temperature cools (say, if it’s exposed to an excess amount of water), its skin will harden into rock and crush its internal organs. Meanwhile, exposing sandygast to water may destabilize its body (even though, yes, it lives right by the ocean—it simply retreats to higher ground during storms and high tide), and the geodude and onix lines should never be soaked, as water can get into the crevices of their bodies and erode their joints.
In short, the rule of thumb is that if a pokémon possesses no external flames and is not made of fire, lava, rock, or sand, they can likely tolerate exposure to water and can be bathed or allowed to swim. They simply might not always like that, and in many cases, be warned that if a fire-type swallows water or exposes its flame sacs to liquids, then it may have difficulty using its fire abilities later on. If, however, the pokémon in question is essentially made of their element, then give them a sand bath and have them avoid water as much as possible.
The Pokédex According to Bill 