You're previous answers are correct, if you have an antenna analyzer it might be a good time to use it. As I looked at it there seemed to be a loading coil in the mast, so it will be a little more complicated than just cutting the rod, it will be shorter, most likely, but if the electrical length may not be the same as the physical length.
I say that because I was thinking of checking the calculator to get the measurement for a quarter wave at 152.5 MHz, then I realized that there may be a difference.
If you can get a ground plane, just a series of wires to counterpoise the vertical, it would help.
Are you going to be close to the salt water, I believe salt water is about as close to an ideal ground we can get. If the antenna is close to the water it should improve performance.
Diamond does say that the warranty may be voided if you make adjustments like that. In addition, this will make the settings for 70 CM change. And since you can't "uncut" an antenna, at least not easily, maybe a homebrew antenna might be a good choice.