There's no great need to dismantle a servo to get motive power - nowadays you can buy model motors that are not much thicker than a typical boat propshaft, and speed controllers the size of a thumbnail. Usually for about a pound. These small drones have really helped in this regard. This one, for instance, is 0.6cm diameter by 1.5cm long - https://www.ebay.co.uk/itm/Original-JJRC-H20-RC-Drone-H20-08-CW-Motor-Spare-Part-For-JJRC-H20-08-RC-He-M4S2/332826605276?hash=item4d7e02f6dc:g:7AIAAOSw989bsxkP:rk:39:pf:0

Your biggest problem will usually be rudder control - there will be little room for a servo and tiller right at the stern of small boats. For an EeZeBilt I recommend a closed loop system which lets you relocate the servo anywhere convenient - see http://eezebilt.tk/radio.html The EeZeBilt Terrier below is 10.75 inches long...