Umm... Clerics don't need to rest for eight hours (at least according to the SRD). "Clerics meditate or pray for their spells. Each cleric must choose a time at which she must spend 1 hour each day in quiet contemplation or supplication to regain her daily allotment of spells. Time spent resting has no effect on whether a cleric can prepare spells."
So you only need the one hour meditation, but it has to be the same time each day (or possibly at a shifting but specific time... high tide, sunrise, moon-set). If something attacks us during that hour (or anything else exciting happens) you have the choice of joining the fight and losing spells for that day, or meditating your way through it. (I am reminded of Robin Hood Prince of Thieves).
We have previously used four two and a half hour watches (and thrown breakfast in as the missing half hour), which I would prefer to five watches since it makes doubling up easier.