It would depend on the 135 lens. If it's a Tessar clone, be very careful with the edges and corners. I am sure your P can easily overdrive the coverage of that type lens. If it's a "w" or"ws" type lens, I am sure it will be fine.
I am not sure I would take my Sinar on a river (if I had a Sinar or a river) but don't let us stop you. A 4x5 Crown would be more my taste. And it would be with the 135mm in this case.
tim in san jose