I would only want to land quarantine in a large property, otherwise you would be cooped up in a small space for at least five days. At least on a boat you're free to move around, swim, etc. The downside on a boat would be bad weather, as you're limited to certain anchorages.

I haven't thought this through, but if you're a large group, you could land quarantine separately. That way, if someone tests positive the whole group isn't quarantined together for two weeks.