Finished, having only started today (busy day yesterday and got very stuck on the Sandwich Sudoku). Neded a few nudges from the forum, thanks- and hadn't spotted the theme but once it was pointed out solved a nice little flurry of clues, some of which almost solved themselves (I only had 1st letter of 16d, but given the theme ...!)
Still puzzled by the "depth symbol" of 12a: in what context?
I had decided 3d was "dockstrike" as matching all 4 crossers and stopping [trade on] water, though should have been "4,5", but baffled by its parsing, till I looked at CrosswordSolver and got the right, nicely clued, answer!
FOI: 21a
LOI: 23d, a new word
COD: let's go for 3d after all!