For a LIPO, each cell is 3.7V

So 6S battery = 6 * 3.7 = 22.2V

for the boat, we need what the motor need:) for example 3 battery 6S it can be perfect, so we have 18S = 66.6V

Capacity: Example 5000mAh =5Ah, so it's mean a power of 5A during 1 Hour, or 10A during 30min... easy

Discharging: 50C it's mean, you can use 50 * 5000mA = 250A, the most Ampere you can have without damage.  

So with 5000mAH, and 250A use, you have 1.2 minute battery... not so much

Charging: Same principle in the other direction. The charge is generally very slow. In our case, we can charge at 5A*3C=15A maximum without damage.