"I can't get over it" mean that you cannot forget something or take your mind off of. It is more often used in the case of a great hurt, such as the death of a loved one or the breakup of a relationship.
Occasionally, you might hear it used for exciting event. Example: Bill is so excited to be invited to the White House, that he can't get over it.
"being at sea" , also seen as "being lost at sea" means you are mentally lost, confused, disoriented. Exmaple: After the breakup of her marriage, Jane had the feeling of being lost at sea.