Code:

let passable gstate (row, col) =
   let t = tile_of_int (gstate.tmap.(row).(col).content) in
      not ((t = `Water) || (t = `Food))
;;

Some might prefer:

let passable gstate (row, col) =
   let landmap = gstate.tmap in
   let lm_row = landmap.(row) in
   let tile = lm_row.(col) in
   let v = tile_of_int tile.content in
      not ((v = `Water) || (v = `Food))
;;

In this case I don't see any benefit in expanding the value-accessing line.

Consider this, though:

let visible_and_passble gstate (row, col) =
   let tile = gstate.tmap.(row).(col) in
   let terrain = tile_of_int tile.content in
   let visible = (tile.seen > 0) in
      visible && (not ((terrain = `Water) || (terrain = `Food)))
;;

Here we refer to 'tile' twice after its initial declaration, so the above could be considered better than:

let visible_and_passble gstate (row, col) =
   let terrain = tile_of_int gstate.tmap.(row).(col).content in
   let visible = (gstate.tmap.(row).(col).seen > 0) in
      visible && (not ((terrain = `Water) || (terrain = `Food)))
;;