Well if you look at our world stats Earth's surface area covered by land is 29.2% that is 57,491,000 area of square miles. Earth's surface area cover by water is 70.8% this is 139,397.000 area of square miles. Their is a lot more room at sea then on land. Granted the perfect set up would be to have a land base retreat and a at sea escape.
If you know what you are doing at sea and have the proper training the risk is very low and may even be safer at sea then on land in a collapse situation.