Nothing's gonna go up your dick.

I do it all the time with my boat

I doubt that there is any physical risk unless the attempt was with dicks that were too big. As with all sexual contact there is the danger from STDs. I would think that you could offer your uncut cock to another male who is cut and give him the sensation of having a foreskin around his bare cockhead.

It depends on the speed of your boat when your approaching the dock

I did have an occasion where i was docking with my BF at the time, and my dick was inside his foreskin and our heads would press together with every stroke, cum slit to cum slit. Well he came but I did not. When i pulled out his cum was dripping out of my slit. At first I thought it was precum but it was real cum and it wasn't mine!

I think it’s highly unlikely anything would travel up your urethra.

It's extremely dangerous if he has a serious STD......

It feels good ,my bf and I have done this a few times ,its great jerking both cockheads with one skin