The nissan SR series motor's would NOT make good boat anchors because they are too light and wouldn't get to the bottom of the ocean quick enough. LOL. Go Karts are regulated to approx. 100cc max capacity, a little more if a rotex is fitted(maybe 150cc).
SR series motors have little torque, they would make towing a tinnie to the local jetty a task not for the faint of heart. A V8 (1uzFE or 2uzFE for easier compliance as my personal preference) from the Japanese barn would be the pick. The 6cyl's from Japan in my opinion also do Not have the required torque output DOWN LOW that would satisfy city driving. Aww maybe a 2J if you must. What about a factory Ecotec SCV6 ? has torque, good aftermarket support, straight bolt in, only downside really is the fact that dyno horsepower figures are lower than 99% of the jap imports. 4 cylinders included.