Is bathing in seawater to hasten the marriage of a delayed girl valid or sorcery?

There is no basis for what was mentioned in the question; rather, it is a form of shirk (polytheism), because it is the belief that something which is not a cause is a cause. Everything that is not a legal (Shar'i) or predetermined (Qadari) cause, if taken as a cause, is considered associating partners with Allah. And whoever believes that something affects by itself without Allah is guilty of major shirk in Tawhid al-Rububiyyah (Oneness of Lordship). As for whoever believes that it is a cause but does not affect by itself, then he is guilty of minor shirk.

Proving effective causes and that something is a cause must be either from the perspective of Islamic law or empirical observation. And whoever is delayed in marriage should take recourse to lawful means, such as supplicating to Allah and maintaining chastity.