Can I watch the full 'Nice Boat' replacement episode anywhere?
The full 28-minute version of Nice Boat is apparently available on Nicovideo (you'll need to make an account or use a redirector like nicoviewer). I didn't see the original Nice Boat back when it aired, so I can't say whether it's the real deal, but it certainly looks right to me. The Nice Boat itself makes its first appearance at 04:36. 

I would be surprised if Nice Boat were ever included in a home video release of School Days, though. At minimum, it's not included in the French DVD boxset or in the 2013 Japanese Blu-ray boxset.