Velvia does make a 100 ISO Slide Film which you may find quite suitable for your marine images. On the downside, I've always felt that Velvia seemed to saturate the greens and blues more so than the warmer colors which is why I've always favored it for landscape images, flora and fauna and not so much for underwater (though I have used it on rare occasions) photography.
It's true that distance plays heavily on the quality of light in the marine environment and you will find that the more interesting photos are taken with either macro or wide angle lenses, the latter requiring much stronger lighting to penetrate the short distance from the lens to the subject.
Water absorbs light... a distance as little as 8 feet can effectively cancel out the red spectrum of light if the strobe you use isn't strong enough to overcome that distance.
I don't know the formula for water but think about this: Light on land conforms to the Inverse square law which in application states something like if you double the distance then take the inverse of the ratio and square it. So moving light 2x further away gives you 1/4th the light hitting any given area. Imagine how much greater water is absorbing your light with it's increased density. Perhaps 4 to 5 times greater which is why color is so affected by depth.
Depth radically bends / absorbs light... first red, then orange, yellow and green. Blue, being one of the longest wavelengths is the last to go but the first you see as you descend in your dive.
Your film, being sensitive to the light spectrum naturally will pick up that spectrum much more accurately than your eyes which correct for these things to a substantial effect. What YOU see is not what your FILM sees (digital or otherwise) but digital can better correct for it in the programming.
Some flashes add a coral hue (orange filament) to attempt to add color to the image. Personally, I hate someone else messing with my equipment (that's my domain... after all, it IS mine) so I will not buy pre-determined and pre-designated flash outfits that suggest to me what I will learn or need to discover.
That is the enjoyment of photography... you keep learning techniques by making mistakes, researching your mistakes and reading the advices of others who preceded you.
I have a Sea & Sea film camera, a Nikonos V (I prefer it to anything else), some video cameras and housings and for a time I was one of the lucky few that owned a Nikonos RS with both available lenses (it got sold to finance a trip to the Philippines).
Photography underwater is completely unique in and of itself as you have mentioned (excellent images out of water with your MX-10) so it would be wise to deal with a pro lab that can make some adjustments to your processing by adding magenta (red) to your film and you too could add that magenta filter to your lens to boost your reds a little (I use it on my video to counteract the blue).
These guys that are helping you have good advice... maybe more than you want or need at your level of learning but it will all come together in due time. Just read the recommended material... apply it, take notes of what you do with your shooting (nothing beats a slate record) and adjust for it on each outing.
Keep as little space between you and your subject as possible (particulates in water can ruin beautiful images when light bounces off them to your lens) and if you can get arms to move your flash further from your camera you won't go wrong. So long as you increase the angle of reflection, you will eliminate particulates on your film... any underwater photographer will validate that statement.
Damn... I'm having so much fun in here !!!