Hello. I played LCS the other day with the goal to get some rare vehicles and I came across Splashproof PCJ. It's the bulletproof bike you get in the "Biker Heat" mission. But to get this special ability you have kill the driver in the precise spot, which is the alleyway shown in the video. The splashproof ability prevents you from drowning.

After getting this bike I would recommend to save it to your garage and pick it up during "Driving Mr. Leone" because it has no use in Portland. However, you can't use it during the mission because the safehouse in Staunton is locked. Once again, I would save it to the garage and wait for some good opportunity.

The first spot is shown in the video and works from Portland -> Staunton and Staunton -> Portland. For the Staunton -> Shoreside just keep going straight until you get to the crossroad near the stadium. Still go straight until you see the tunnel on the minimap. The road turns left but you have to turn right into the water so the tunnel is on your left side. Be very careful not to be too close to it or you will get teleported into it and there's no escape out of it. Also I haven't found any spot for Shoreside -> Staunton but you can use the bike to avoid bad bridge timing.

Here's jam8tone video, he explains much more better than me:

EDIT: Found a way from Shoreside -> Staunton. Just go to airport and take the first runway.