ETR3(SS) and jhapprich are two "modern" sub modders. Search on one of their names by clicking on the "Downloads" link on the menu bar above, then on "Search" under the "Navigation" menu on that page, and you should get several hits. Both are still active on here. I've not kept up with the mods, but there are several mods, but some are for v1.4 of SH4, so you might be better off waiting for one or the other to post, or find their "Release" thread and ask there, as to which is appropriate for v1.5 of the game.
As for the camera, have you tried using the period <.> and comma <,> keys yet? That gets you the Next and Previous units on the external camera. If there isn't another vessel close by, it will show your boat, whether surfaced or submerged, from a little ways out and slightly above. Move the mouse around, and it rotates and you can zoom with the arrow keys (and mouse wheel??). The "camera" moves around a bit too fast, and doesn't allow you to zoom in too close, but you might find it useful...
