While it is technically permitted to immerse in the sea, however there are many potential issues and you must consult with a Rav who is an expert in hilchos mikvaos. Certainly it is preferable that she should travel to the nearest Kosher mikva, as this will resolve all the potential issues.
Furthermore, one may not immerse in a place where there is concern for safety or privacy out of concern that she will do a rushed and hasty job and not immerse fully. If it is unsafe to immerse at night but during daylight it is safe and private, than under extenuating circumstances one may immerse on the eighth day or later, meaning a day following the night she would usually immerse, but not preceding.
Also, a woman may not immerse on her own out of concern that she may not submerge all of her hair under the water, and it is problematic having the husband supervise because he generally may not see his wife undressed before she immerses in the mikva. An alternative is to loosely bind her hair while she immerses by herself, taking great care that the entire ponytail is fully submerged. This, of course, potentially extenuates a situation where she will be nervous being alone on the beach at night.