safe sure, just not ideal, especially in a cave. This is what you have to remember about visibility. A good light, no matter how bright it is, or how tight it is, can not make visibility better. It is either there or it isn't. If the vis is 5" because the particulate is so dense, no amount of light will allow you to see through it. In a cave, the big head will be less than ideal, but is what it is. I would recommend going on Cave Divers Forum and perusing the used light section. UWLD-13 and UWLD-15 have tighter beams which are nice, and you can find HID's for pretty cheap on there.
Here are a few problems with that light for what you want. It is too much light for murky water where you will get a lot of backscatter problems. You can't turn it down, and it likely isn't tight enough of a beam to not induce significant backscatter. The low voltage battery pack is also a problem because as the pack voltage drops, so will your total light output, especially towards the end of the pack life. It's a good light for the price, really is, but it won't fix your low vis problem, and it isn't ideal for cave diving. Just my opinion though, plenty of guys are happy with it