Can I go to the sea or to the beaches?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Going to beaches and sea for a walk, picnic, swimming, etc. is permissible except if the place contains abominable acts such as nudity, mixing men and women and practice of evils, etc. which is common in some parks, then it is not permissible to be there.
However, it is permissible to visit those places for the sake of Da'wah (calling people to Islam) and to prevent them from evil if one is sure that he can control himself from being attracted to sin.
Allah knows best.