I think they have an invitation-only industry forum here- see if you can contact one of the moderators.
My random thoughts on it all:
Be sure and try riding the trikes on the sand. It might work great or might not, depending on how loose it is. For rental, you'd probably want something a lot heavier-duty than what you're buying.
Worksman (and some others) sell reverse trike ice-cream-carts that actually come with an umbrella. I'm sure they could mount an umbrella on a reverse-trike delivery trike (which just has a big cabinet) as well. But trying to ride with one of these might be a challenge where you have numerous trees or when it's windy. If it's windy enough, they could probably blow right over while unattended. But the delivery-trikes with a big box would work great for storing your stuff while at the beach. www.worksmancycles.com
I would not have them remove the seat, just have a separate post mount for the umbrella.
The reverse tricycles look a lot cooler than the normal "granny cycles" (what somebody else called them). But they do cost more and are tippier.
On the adult tricycles, you can also get several different brands of "industrial" tricyle or even quadracycle. You can also get surreys which are intended for the rental market. All the trikes are intended for 1-person only unless they're a pedicab. Seems like I've read that the surreys are more for novelty use, like the pedal bumper-boats they rent, and don't work that well as transportation.
You can get pedicab trailers that mount to a regular bicycle. Perhaps mount one of those behind a granny trike.
For the application you describe, pedicab rentals might actually work pretty well, and would be more fun to rent than the trikes (assuming you mean to just rent them out and not operate a service). Downside: Good ones are expensive, and the bigger they are, the harder it is to run them down bike paths or beside road traffic. Also, just one person pedals, so that one person better be a good cyclist or they're not going far.
They have some European pedicabs with semi-enclosed compartment that would look cool. Blow-over in the wind would still be an issue, as would cost.
Bakfiets would be another option. Carry two people or an adult and a couple of kids plus some junk. I've never ridden one, so don't know if there's any special skill to be learned. And they are expensive.
The Worksman and some of the other heavy-duty stuff is all good sturdy steel, but steel and beaches are not an ideal combination due to eventual rust.
How long is the "beach season" there? A short season would make it harder to recover your costs.
If you contact some of these manufacturers, ask them for any references in your general area. This might let you go try one of the vehicles without buying, also let you talk to the owners and see what they think of them.
You can store a bunch of bikes in a garage. But pedicabs take up a lot of room to store.
I remember seeing a post on Craigslist where someone was running a bike rental business somewhere in the US. During the offseason, their storage place was broken into and all the bikes stolen. So don't forget to add insurance (including liability) into your costs.