Glad you asked…
GoPro 3 Red Filter – http://stuntcams.com/shop/gopro-hero-dive-underwater-color-correction-filter-p-577.html
Inexpensive and effective, but requires an install
You can use this with your 3+ case: http://stuntcams.com/shop/gopro-oculus-color-correction-underwater-dive-filter-p-595.html
It’s cut for the 3 case but will still work and pull tight over the 3+ model.
We also offer the red filter with a dive housing, which is better than the 3+ case, and is rated for deeper dives:
http://stuntcams.com/shop/gopro-hero-dive-filter-plus-dive-housing-p-739.html
3rd solution:
http://stuntcams.com/shop/gopro-oculus-filter-plus-dive-housing-p-738.html – Best housing with oculus filter.
I would also go to a dive shop and have them test the item if you don’t want to waste a dive on your trip, so they can pressure check it for you. Even though the cases are ‘waterproof’, GoPro will not cover a soaked camera, even if it was their case in the original state…