Raynor
Bullet-biting time.
25mm props are way too small for this power train. I run that size prop on a Speed 400 in a 20" all-balsa Swordsman. With the weight of two 6v 12AH gel-cells as well (2 Kg each) then your model will do little better than sit there and slowly whisk the pond-water. The Decaperms are geared motors, so you would probably do better with something like 45mm-50mm 3-blade props.
Using two SLA batteries connected in parallel there's every possibiilty that one is discharging into the other and so starving your motors of power and slowly damaging both batteries.
Steer clear of LiPo batteries unless you fully understand the correct way to operate them - they are NOT fit-and-forget items. Be aware that, at best, you'll only get betwen 100 and 150 charge/discharge cycles out of them, whereas you'll get nearer 1000 cycles from NiMH cells. The only real advantage of LiPos over NiHM cells is weight, but there's no need to worry about that too much once you've removed those heavy SLA bricks. 4600 mAH NiMH Sub-C packs is probably the best option here.
The best place for resistive speed controllers like those you have is in the bin - or maybe the Science Museum. They waste power like nothing else I know, apart from supermarkets who insist on fully lighting their stores all night.
Suit yourself, but change the props, speed controllers and batteries at the very least.
FLJ