costs are not a factor in using LiFePO4 packs.
Well, I guess when it comes to boat ownership, costs obviously aren't a factor or no one would ever buy one.
The biggest LiFePO4 battery I could find with a price attached is this one from eBay
That's a 48V, 20Ah pack that sells for $429 FOB China (plus $95 shipping).
I can't vouch for the quality though -- as Red Green says "Nothing says 'poor craftsmanship' like lumpy duct tape."
Rewired this would be 12V @ 80 Ah. Similar to a Group 24 lead acid battery. Quoted weight is 20 lbs vs. 45 for a Group 24 lead acid. So you could save 25 pounds per battery.
A cheap lead acid is maybe $75 at your local big box store.
These things have a nominal cell voltage of 3.60 V, so four cells in series will give 13.4 V, a little bit above regular lead acid. Charging voltage is 4.20 V per cell, so a 12V pack would need a charging voltage of 16.8 V. Standard alternators are regulated at about 14.1 V, so it appears they couldn't charge one of these LiFePO4 batteries.
When mass production brings the prices down and provides the charging technology, LiFePO4 will probably give lead acid a run for its money. At current prices, installing them on a boat would just be another expensive hobby.