DSLR Lenses are designed for relatively large CCDs (up to 24 x 36mm for full frame cameras). The iPhone has a tiny CCD and needs a much shorter focal length to give you a normal field of view.

You would need an super wide angle lens to achieve normal viewing angles with this device. Any normal DSLR lens would just give you a super telephoto lens. This is already the case for consumer grade DSLRs where you often have a conversion factor of 1.6 or 1.5: A 50mm (normal) lens takes photos as a 75mm (light tele) on a 35mm film camera.

The smaller CCD effectively crops your image. This is cool if you're into wild-life photography and very cool as the image only sues the centre part of the image circle of the lens (for lenses that work with both half and full-frame cameras) because every lens degrades close to the edge of the field of view. However, if you need a 20mm wide angle lens you have to get a very expensive 14mm lens for the same effect.

I fully agree with that, that's why I suggested to join the local grotto / cave club before going off exploring in the original post. I probably could have emphasised this a bit more.

Most of the flotsam there consists of small particles that are distributed in the first 10m of the water column. What would need to be done is to filter it out and bind it similar to how pebbles are bound with cement to create concrete to create large enough bits that can be combined into an island.

Eventually we (the world community) will have to clear this patch as the plastics now enter the food chain and threaten to poison us all. Already there are areas in the ocean where plastic is more prevalent than krill and plastic is being ingested by marine animals, accumulating in higher organisms and ultimately in us too.

Collecting plastic there would be a nice occupation for all those fishermen that have been made redundant due to overfishing and the necessities to conserve fish stocks. Get them to fish plastic instead and pay them for the trash catch they return.

Two articles on that matter, a bit lengthy but worth your time:
http://www.orionmagazine.org/index.php/articles/article/270
http://www.mindfully.org/Plastic/Ocean/Moore-Trashed-PacificNov03.htm

Exploring caves is the last adventure left to the proverbial "common man". Everything is mapped and surveyed except caves. Even if you climb a mountain as a first ascent, someone has photographed it and its height is known. There is no technology that allows to survey caves without going there and that is the excitement and fun of it. You can do it big as Bill Stone of you can find a few meters in a local cave and you can do it according your technical and physical ability. Just join the local Grotto and you have that chance! Nothing beats entering a passage where no other human being has walked before and where your light illuminates formations that nobody has seen before. You can do this only in space and on the bottom of the ocean but the costs and technology needed for that is beyond the reach of hobbyists.

There will never be the ultimate deepest cave as we know the highest mountain as there are no means of knowing this until all caves are explored. Estimates place the ratio of explored caves at some 5% of total caves. Some have not even an entrance... Of course, we know the theoretical limit which is the height difference of the limestone bedding that houses the cave but there might always be a higher entrance or a sump or something else

The reason why caving is not as popular with viewers is that it really is not a spectator sport. All you see is some cavers departing into a deep hole. Comparing this to seeing mountaineers where you can see the mountain, the cliff and where you can admire the challenge you have no such chance with a cave. And if you're not a caver you can not imagine the challenge, the joy, the cold and the misery and the excitement.