The Watersnake SALTWATER ASP T18 & T24 electric motors, are low cost, light easy to use and very reliable. These little motors offer far more power than you would expect a first glance with the mini two blade propeller. However, as a rule of 'thumb' 1HP equals around 7lbs thrust. Suited to kayaks, small dingy and inflatables.
Complete with stowaway transom tiller handle & switch, the motors are ready to go. Just add a sports 12V battery.
In addition to the transom mount. Watersnake have created a side mount kayak bracket which will fit almost any make of kayak. Battery advice: Amp Draw 18lb = 7-15 amps & 24lb = about 9-20 amps
First it needs to be a 12V sports battery (designed to discharge to low /empty and recharge, unlike a car battery designed to trickle from car engine)
The higher amp/hr rating will give you more power time, but it's a balance, you need to decide what is practical to put in your kayak or boat. My advice is go to Halfords or any battery outlet and look at them there.