If you somehow visit Swinging Ship after the first combat phase of a turn has ended, it won’t have any effect. No additional combat phase will be created.

If you visit Swinging Ship multiple times before the first combat phase of a turn has ended, that many additional combat phases will happen, one at a time. At the beginning of each of those combats, any creature that attacked during any combat that turn will untap.

Swinging Ship won’t untap any creatures that entered the battlefield attacking. Those creatures never attacked, meaning they were never declared as attacking creatures.

If you somehow visit Swinging Ship during an opponent’s turn before their first combat phase ends, they will get an additional combat phase. Their creatures that attacked will untap.