My boat had a Shakespeare FM band splitter in line with the VHF radio
antenna. I can't tell you how much it may or may not have affected the VHF radio receive or transmit strength. Properly designed I think it shouldn't make very much difference, but there will be some insertion loss - though it may be small. I removed it just to take one more variable out of the loop.
To replace it, I used a simple dipole mounted in the cabin
behind the panel. These are easy to make, or very cheap
to buy and can be found in many places. Reception
may not be quite as good with the little dipole, and will be more directional, but in practical use I find little difference.