eTwee wrote:I'm a bit wary of LiPo however. Heard nasty things about them?
They can cause fires if punctured (which shouldn't happen if they are properly stored in the kayak). You also need a protection circuit; this can either be in the battery or separate from it. This is not just to prevent short-circults causing fires but also to prevent over-discharge. If a unprotected lithium chemistry battery is over-discharged even once it will be permanently damaged; sometimes they can be recovered but it will always do some harm. The protection circuit will turn the output off when it reaches the minimum voltage (this is why LiIon etc battery packs seem to have a sudden threshold where they run out of juice; it's the output being cut off, not the battery being completely discharged).
Additionally it is good practice to never leave high-power LiPo/LiIon packs alone when charging and to use a fireproof charging sack to hold them during charge.
Note that it's common for LiPo's intended for radio-control devices to not have protection in the battery itself as it is expected to be present in the ESC for the RC model. If the battery pack doesn't explicitly state it has protection you should assume it does not unless told otherwise by the vendor.
Personally if I were putting lithium batteries into a yak I'd use LiFePO4 rather than LiIon or LiPo, but if doing the latter as long as it has a protection circuit and is properly mounted and charged you can get away with it.