Additionally, how did sailors on the open seas get fresh water?

They didn't. Ships would take on barrels of water when they left and when they stopped along the way. But, as you can imagine, after a few months at sea the water would be less than fresh.

Apart from water the crew would also get rations of beer and, to a lesser extent, wine, though, which should keep a bit better. This makes me wonder though, how they coped on Islamic ships.