Well you longboard surf and have just gone out and bought what the guy at the skate shop assured you was a longboard. With extreme excitment you have pushed off and found that there is no where to walk and the things to narrow to do anything on. Sorry but you should have read this first :-)
Longboards now come in all sorts of shapes and sizes. Some are just plain crap while others are made for a specific purpose. Not may are made to simulate Longboard surfing at all. So what should you look for ?
Here is a checklist to help you decide if a board is suitable to simulate your longboard surfing on :
Length - Here it should be at least 47/48" plus. Ideally it should be 53"plus. I rekon the ideal is at least 58". Still Stoked Surf trainer is 72" !
It should be flat with very little flex for your weight.
It should be wide ... no use having a skinny deck you can't move around on. If you have big feet go for around 11" if not then 9 1/2" should just do.
It shouldn't taper off to a pintail at the front. You need a rounded nose which is almost the widest point of the board. The tail can taper off slightly on longer (53"+) boards as you will not need it much, I still prefer a rounded tail but not as fat as te nose. But with smaller boards you need a square or rounded tail not a pintail.
You don't need a kicktail or upturned nose on a board over 52". On 48"-52" a kick is Ok, I prefer a flat nose or one with a slight upturn. On some boards you get a slight rocker all the way through which is nice on smaller boards.
Wheel cutouts are OK but I avoid them. Slight ones help to stop wheel rub but if you use new style trucks yo don't need them and they can be a pain if you are turning with your feet just in the wrong position as they will catch your shoes. An example of this is the 48" gravity boards which are a pain to nose ride because of the deep wheel cut outs. (Their Classics get over this though).
While I think it is an individual choice here are some decks (and links to the sites if they have them) I think are good for longboard surf simulation.
STILL STOKED - English boards that are made for longboard surfers
Black 29 - their 60" Royale model
Tahoe - their 62" model
Gravity - Their 52" Classic
Fluid skates - larger ones
Kapu skates - the noserider and bigger model
Topps - if you can get them
Real Wood - pity they are only 48"
Vision - Their 50" longboard
Powell - their 50" Pele model
Envy 57" - never tried one but they seem good
This is NOT to say that other boards are crap, the above I have got and people like Sector 9 just haven't got their big board act together (just step on a luke Nosewalker).