for a boat the size of yours the simplest way to get the current needed but still have a small a light battery is la ithium polymer battery. Lipo battery packs are made up of cells each with an average of 3.7v. We call each cell S (dunno why). 1s battery is a 1 cell battery, a 2s battery is a 2 cell, 3s 3 cell etc etc. For your boat you will need a 2s battery, itl have 2 cells connecting to each other making your pack a total of 7.4v.
Lipo batteries create much more current than theyre equivalent size counterparts so are ideal where a lot of power is needed but not too much room or weight to spare. Lipos however need to be respected and operated correctly otherwise they can fail on you and sometimes even catastrophically. They need to be charged with a balance charger, Its a must and not optional, it chargers the cells individually in the pack and once each cell has got to 4.2v the charger cuts off, beyond this the battery is damaged. On the other hand when the battery is being discharged in the boat the voltage must not drop below the 3v percell or itl damage the battery too. This is where the low voltage alarm comes in, it sounds off when the batteries in your boat have reach 3v percell. When the sounder goes off you bring the boat in. simple
Right here is a lipo ill recommend for you:
the 2 pence peices and my lighter gives you an idea of the size. The item to the right is a lipo alarm this will work for batteries up to 4 cells. The item at 2 oclock is a lipo voltage checker and you will need this to keeo tabs on the individual cell voltages and total voltage. you buy this battery from here:
http://www.giantshark.co.uk/gens-2s1p-lipo-battery-p-406444.htmlits connector will plug straight into your esc.
you will need a balance charger for it:
http://www.ebay.co.uk/itm/130761623057?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2648this will charge both 2 and 3 cell batteries, you plug it in like this to your battery:
you will need an adapter to power the charger:
http://www.ebay.co.uk/itm/200812742842?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2648i use this adapter, dont use the chinese ones, i had one and it turned into smoke.
voltage checker:
http://www.ebay.co.uk/itm/RC-Li-Po-Battery-Voltage-Indicator-Checker-Tester-2S-6S-/220755302516?pt=LH_DefaultDomain_3&hash=item33660a9874low voltage alarm:
http://www.ebay.co.uk/itm/2-3s-7-4v-11v-RC-Lipo-Battery-Low-Voltage-Alarm-Buzzer-Indicator-Tester-Model-UK-/320935572815?pt=UK_ToysGames_RadioControlled_JN&hash=item4ab940354fThis is how you connet it up in your boat. The boat im showing it on has space for aa cells cos lipos are too fast it being a canal boat:
connected to voltage checker: