--Let's( )in the river!--No,it's dangerous.是swim还是swimming

--Let's( )in the river!--No,it's dangerous.是swim还是swimming
--Let's( )in the river!--No,it's dangerous.
是swim还是swimming

--Let's( )in the river!--No,it's dangerous.是swim还是swimming
swim
let's do用动词原形.

swim
Let 后面都要加原形的

根据后面的回答可知此时是一般现在时，用swim