I’m using CALB 72AH cells http://en.calb.cn/product/show/?id-630 with a WATE 4S 50A PCM Protection Circuit Module ($21) I found on Aliexpress. CALB USA is in Southern California and I was able to get dealer pricing without too much trouble, saving me 30%.
LiFePO4 has a lower cell voltage than Li-Poly, but much better longevity and safety. The CALB 72AH can put out over 200 AMPs.
4 CALB cells gives me 72AH 14.4V. My craft is a catamaran and I use two battery packs (144AH at 14.4V). Each pack powers a pair of T200s with BlueESCs. My cost for each pack is less than $500.
I’d have liked to up the voltage, but a 5S LiFePO4 PCM is really hard to find as is the charger. As Phil mentioned, a non-planing hull has a max speed and I hit it without needing 18V. Can’t stress enough how important it is to measure hull speed at various throttle settings to maximize battery life. I put code in to re-normalize the throttle to not go beyond the max. More wattage than you need is just wasted.