Game of Thrones: ship rules
From the rulebook:


No. "Ship units may move into friendly connected port areas or into adjacent sea areas, but may never move into a land area." (p. 15) So a ship alone cannot attack or conquer land. It can support a land attack, though, although the reverse is not true (p. 18).
As indicated above, this cannot happen.
You can capture the ships. "If the land area connected to a Port is successfully attacked and taken control of by another player, that player may immediately replace any enemy Ship units in the Port with an equal number of his own available Ship units (or as many Ship units as he has available, he desires, or is allowed by supply limits). Excess enemy ships are simply removed from the board." (p. 25)