Welcome to DrRacket, version 6.3 [3m].
Language: Pretty Big; memory limit: 128 MB.

> cOcean
((- - - - - - - - -) (- - - - - - - - -) (- - - - - - - - -) (B B B - - - - - -) (- - - - - - - - -) (A A A A D D - - -))

> (markchar 1 5 'M cOcean)
((- - - - M - - - -) (- - - - - - - - -) (- - - - - - - - -) (B B B - - - - - -) (- - - - - - - - -) (A A A A D D - - -))

> (markchar 3 5 'F cOcean)
((- - - - - - - - -) (- - - - - - - - -) (- - - - F - - - -) (B B B - - - - - -) (- - - - - - - - -) (A A A A D D - - -))

> (markchar 24 45 'G cOcean)
#f

> cOcean
((- - - - - - - - -) (- - - - - - - - -) (- - - - - - - - -) (B B B - - - - - -) (- - - - - - - - -) (A A A A D D - - -))
> ;; Note that markchar never CHANGED (that is, with set!) the passed in board itself...it just returned the updated
;; board (which is just a nested list!) as its return result to the caller. It's up to the caller to do something with it.