Underwater filmmaking

because the lens specification is changing underwater.(everything looks bigger)
if you do not plan this by correcting the lens, you end up with shots that are not stitchable.
(probably there is not enough picture overlap left, or the geometry of picture is too much distorded to be usefu).
the easiest way to keep lens specs underwater is to use a dome port versus a flat port.

probably , the underwater version also has lens corrected to be used underwater.
they probably built the camera around this
http://www.amazon.com/KNEKT-Six-Inch-GoPro-Hero3-Hero4/dp/B00S66S90I

so, even if you need to buy 6 domes like this , you are still a lot cheaper than the 5000$ tag.

you can even go cheaper with this
http://www.amazon.com/gp/product/B018279SA0/ref=pd_lpo_sbs_dp_ss_2?pf_rd_p=1944687622&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B00S66S90I&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1XGK88Q4924MD2H21DV1