Well here goes nothing. I will try to explain this as best I can.
Get yourself a PCJ 600 (I have done this with a couple of the faster cars as well, but a PCJ 600 works best)
Ride up the road coloured in red on the map at high speed (Ie lean forward to make the bike go faster).
As you ride up you will see the business building that you own, but it will not be fully loaded, ie a dark grey square box because yet again, you have outrun the graphics.
Avoid the wall and ride straight at the building. If it has not loaded then you will ride straight into it as Neon™ pointed out in a previous post. Stop inside the building but get to the back of it and wait on your bike. The building will (eventually) load and you will fall into Grey Hell. After the wierd pause you will fall into water, and be able to swim around under the map. you must stick to the edges though as the water does not fully go under the map.
I have been able to get into the inside of most business buildings you can get a half decent run up at, although I have not been able to get into any with water underneath as yet.
Just one other thing to note. You have to get the the back of this building to be able to fall into the water. If you don't and miss the water you will fall into nothing before it re-spawns you in...... The Mall. Strange or what. If that happens you will have to reload as the game does not recognise you being in The Mall so it will not give you the blue arrows to get out.