1) The children couldn’t go swimming because the sea was rough.
=> The sea was too rough for the children to go swimming.
(OR: The sea was so rough that the children couldn't go swimming.)
2) You can’t visit the USA unless you have a visa.
=> If you don't have a visa, you can't visit the USA.
3) Last night I had dinner at home. Then, I went out with my friends.
=> Last night, I went out with my friends after I had had dinner.
(OR: Last night, I had had dinner at home before I went out with my friends.)