Hi Boom
Long time no see...that's life...
Yep you know we can't add any type of engines without crashing BfV
The only working thing is the nitrous
I tested this one for your sheridan:
ObjectTemplate.Active Sheridan
ObjectTemplate.hpLostWhileDamageFromWater 0
ObjectTemplate.addTemplate Chinook_Floater
ObjectTemplate.setPosition 2.5/1.4/-2.5
ObjectTemplate.addTemplate Chinook_Floater
ObjectTemplate.setPosition -2.5/1.4/-2.5
ObjectTemplate.addTemplate Chinook_Floater
ObjectTemplate.setPosition 2.5/1.4/2.5
ObjectTemplate.addTemplate Chinook_Floater
ObjectTemplate.setPosition -2.5/1.4/2.5
ObjectTemplate.speedmod 0.5
ObjectTemplate.addTemplate PT76_Rudder_Stern
ObjectTemplate.setPosition 0/-1.75/2.5
ObjectTemplate.setRotation 0/0/-90
ObjectTemplate.addTemplate PT76_Rudder_Aft
ObjectTemplate.setPosition 0/-1.75/-2.5
ObjectTemplate.setRotation 0/0/-90
ObjectTemplate.addTemplate PT76_Floater
ObjectTemplate.setPosition 1.6/1.4/-2.5
ObjectTemplate.addTemplate PT76_Floater
ObjectTemplate.setPosition -1.6/1.4/-2.5
ObjectTemplate.addTemplate PT76_Floater
ObjectTemplate.setPosition 1.6/1.4/2.5
ObjectTemplate.addTemplate PT76_Floater
ObjectTemplate.setPosition -1.6/1.4/2.5
ObjectTemplate.addTemplate e_WaterBackBTR60
ObjectTemplate.setPosition 0/-0.492/-3.811
ObjectTemplate.setRotation 0/0/0
ObjectTemplate.addTemplate NancyHorn
ObjectTemplate.setRotation 180/0/0
ObjectTemplate.Active SheridanGunBarrel
ObjectTemplate.fireingForce 0
ObjectTemplate.recoilSpeed 0
ObjectTemplate.recoilSize 0
ObjectTemplate.active NancyHorn
ObjectTemplate.InputFire c_PIAltFire
ObjectTemplate.reloadtime 0.01
ObjectTemplate.roundOfFire 10
ObjectTemplate.fireOnce 0
ObjectTemplate.fireingForce 15
ObjectTemplate.recoilSpeed 5
ObjectTemplate.recoilSize 5
Right click while in water and the tank will move (it will almost not move on lands)
Maybe you can delete the gunbarrel thing cause the nitrous has enough force in water(i didn't try)
You can play at will with nitrous values of course
Any non amphibious vehicle can move in water with floaters, wings and smooth nitrous
I made a jetsky with scooter but i left that crazy idea
I didn't remember i posted that... 
What do i stop now? drink? smoke? both? 