There are actually three detonations shown in the https://cdn0.vox-cdn.com/thumbor/MZpSIvZFx0oZGh9y1sdafVcigPU=/cdn0.vox-cdn.com/uploads/chorus_asset/file/3791408/Fallout4_Concept_Blast_1434323459.0.jpg at the end of the E3 showcase. The positioning of each detonation would put the depicted street somewhere in the region of https://www.google.co.uk/maps/place/Newton,+MA,+USA/@42.3253729,-71.2136779,19221m/data=http://forums.bethsoft.com/topic/1526246-the-glowing-sea/!3m1!1e3!4m2!3m1!1s0x89e38209840725a5:0x5592087b9b70af7e, with the rightmost detonation occurring below sea level (due to the height of the mushroom cloud) in the water just East of the https://www.google.co.uk/maps/place/Boston+Harbor+Hotel/@42.3562236,-71.0502553,4803m/data=!3m1!1e3!4m2!3m1!1s0x89e370875bdec323:0xe6463ad3304ba3cb. The damage to sea-facing buildings can be seen http://i.imgur.com/RrGV64T.png, with the hotel being located near the arch in the foreground. Note that the area around Scollay Square (as shown in the trailer) can be seen in the background to the left of the image, and the detonation from the ground impact in the https://www.google.co.uk/maps/place/Prospect+Hill,+Somerville,+MA,+USA/@42.3835755,-71.095343,4801m/data=!3m1!1e3!4m2!3m1!1s0x89e370cc9331544f:0x92cd86c6ff3ab428!6m1!1e1 area can be seen to the right. The third impact site, as shown in the trailer and the tutorial, will be in the area of https://www.google.co.uk/maps/place/Lincoln,+MA,+USA/@42.4288801,-71.3163945,19189m/data=!3m1!1e3!4m2!3m1!1s0x89e39b0c0c3832c1:0xec99885f78a850e6. Out of these three blast sites, the most likely candidate for the'Glowing Sea' is therefore the one in the harbor itself.
Even if not directly tied to the Marine Boy/Marine Girl perk, there is a very high likelihood of additional V.A.T.S. and HUD functionality when underwater. This is due to the strong theme of water, as shown through multiple clues including the name of one of the recently released audio tracks, which is called 'The Last Mariner'. Note that there are also several underwater images shown in concept art, and so underwater exploration will most certainly be an extensive feature. As for the location of the Glowing Sea, this has only ever been suggested, but never actually confirmed. See my notes above for further explanation.