Hi Scruff

Welcome to the forum! I agree with Eric that the empty water bottles make good reserve buoyancy. In fact, I have designed my entire water stowage system around that principle, with each empty fitting back into a sealable locker. Bottles also mean you can move the full ones around to help with trim.

Other comments about using saltwater hold good - as you know many yachts have a saltwater pump in the galley. According to a well-known designer, when it comes to weight, the minimum you should allow for (water, food and other consumables) is around 10lbs ( 4.5kg) per person, per day. (This is why micro-yachts struggle with long range trips.)

The crew of Lively Lady (currently circumnavigating) have been using personalised bottles for their water, even though they have a tank, so they can keep a track of how much each person is actually drinking, especially in the hotter climes. (I think they have to get through at least 1 litre bottle each per day - in addition to tea, soup etc).

One boat I know of sailed from New Zealand to Alaska, non-stop, and kept the tanks topped off by catching rain! Personally, I'm going to go with the old addage of a gallon (4.45lt) per day, per crewman, which builds in Erics safety margin. Thankfully, after a slow start, the boat will get lighter by 11lbs every day!

Talking of weight, if the volume of water that would fill the average fridge freezer weighs about a ton, it makes sense to fill in as much of the cockpit as possible....