try the pmr sets quite cheap, i think they have eight channels and some can be coded so you have a little bit of privacy. You can add as many as you like, we use them for the kids so they can go off and explore. ideal christmas present so get the wife to add it to your list....
Halfords and many other outlets sell PMR's at cheap prices .... I got my Binatone PMR pair at Halfords at 39.99 a set. They are good for 3km's and pretty rugged. Included was hands free for each as well .... I wouldn't think they are waterproof, but likely splashproof ...
I use mine round the estate and also plan to carry on boat ..... even on a 25ft'r there are times the engine hides voice to bow etc. the odd excursion ashore by SWMBO can be still in contact !!
We have three sets and they are dead useful even if not waterproof (we put them in aquapacs if necessary, just like vhfs!) Got our extremely cheap at Lidls and they work fine. Particularly good when on opposite sides of hull or opposite ends of the mast, to reduce shouting!
If you want a real waterproof 2-way radio, it will be expensive. On the other hand, there are a few "water resistant" PMR radios around. Cobra seems to be a popular brand, and <A target="_blank" HREF=http://www.maplin.co.uk/Module.aspx?ModuleNo=38085&TabID=1&source=14&doy=9m10>Maplin</A> sell a twin pack for £50, which might be worth considering. Otherwise, West Marine have a Uniden with JIS4 water resistance for $100 a pair, but of course these would be technically illegal to use in UK.....
Cheap and legal two-way radios were all more or less just toys until the PMR 446 format became legal a few years ago. As has already been suggested, the more advanced ones have the basic 8 channels sub-devided into 38 subdivisions giving about 300 channels. We have been using a couple of pairs of Icom 446's round the yard for several years now, and I wonder how we ever managed without them. We very rarely ever hear anyone else on 'our' channel.
PMR's from Maplins / local electrical shops give free communications - once you pay for sets and batterys of course !!
IN my case - wife and I have foreign GSM Accounts - so using a mobile as suggested here is a total no-no ....... PMR's - they work great and can always be put in an Aquapac if necessary. (A zip-lock sandwich bag will suffice as temp job ....)
If you really want to have wide ranging advantage .... a couple of second-hand Hand-Held VHF's are the answer .... but of course you already ruled them out in original post ....