Garden of stones, that are similar to watermelons
On the type of rockgarden described, you can:


Reduce the slippage tendency by loosening your pressure on front brake. Let's put it this way: the stone surface provides you with little friction. In some circumstances, the friction is just enough to keep the tire rolling in line. Add the friction needed for braking and you overpass the available friction. The tire then will slide to wherever the sum of all other forces lead it.
You can predict where the tire will go when it slips. It will usually go to the gap between two rocks. Thus it is useful to pick a line where you follow the natural trace given by those gaps.
Ignore slippage. This is indeed is easier said than done, it sounds just weird, I know. The point is, if you pick a good line, specially a straight one, the inertia of your body mass is bigger than the one from your bike. This means that your body tends to go in a straight line, so even if your bike slips a few centimeters below you, it will not be enough to immediately tip you off the bike. You will have a fraction of time to react and return to the balance point. For this to work, your bod must be traveling in the direction of the line you have selected.


Finally, the general idea is to feel like you "float" over the rock garden, get used to the feel of the bike sweeping a couple of inches below you while keeping a good, firm but not stiff grip on the handlebars.

If you don't feel comfortable at certain speed, reduce it, and practice. Have patience. Sooner or later your speed will grow and you will master these sections. Also, with practice, your line selection abilities will improve, you will do it faster and find ways of sticking your front wheel in strategic points where it wont slip and those points will serve you to improve control, switch lines or change course. Remember: practice and patience. 

On a side note: Did you ever as a kid played running and suddenly stiffening your feet so you slid on the floor for a couple of yards? You weren't afraid of such slippage right? Why? You already knew how to predict it, you actually enjoyed it, so relax and enjoy your gardens!