var tx, ty, ts, ts2;
ts = 16 // tile size, in pixels
ts2 = ts / 2 // half-size
for (tx = 0; tx < room_width; tx += ts)
for (ty = 0; ty < room_height; ty += ts)
{
    // if there isn't anything at this position (checking at tile center):
    // (could also use !place_meeting if all land objects have the same parent)
    if (position_empty(tx + ts2, ty + ts2)) {
        instance_create(tx, ty, obj_sea)
    }
}