The Icom M87 has PMR like the Horizon but as with this unit you will need a PMR licence they are not for licence free channels (these are UHF)
Just before I went on holiday I went to see a company called Starcom
They do bike to bike kits and are looking to interface them with Icom Marine kit.
Mainly as they have fitted some of the Honda F4S boats this season.
Their kit is all waterproof and well priced.
They can also do multi station kits for larger boats.
There is also D S Developments, they do a lot of the kit for the RNLI.
Greytronics is great kit but like all great kit its not cheap!!
All MOD boats with intercoms use Greytronics, fitted as standard by Halmatic.
I have tried all of the above kit with Gecko's as well as the DS add on unit for the helmate that plugs direct into our M1V and M87e (boom mic and ear piece that fixes to the helmate)
Gecko do their own but have not tried theirs as yet.
This I use the DS one most weekends.