there must be something wrong with your stroke count, your TT or your stop watch. If you have 18SPL constant, on your first lap you'll have to add 2beeps (first arm-entry on third beep), on your second-third-fourth lap you'll have to add 3 beeps for turns and push offs. So for 100m you'll need
83beeps x 1.2sec/beep= 100sec=1min40sec
or if you offer yourself a beep more for taking up
(4x18+3+3x4)beeps = 87beeps
87beeps x 1.2s/beep = 104sec = 1min44sec
With 1.0sec/beep you are at 1min27sec for 100m and this will be very fast for many forum members (me included).
Most of us have to face there are most time other swimmers with more or less worse strokes who are nevertheless faster than we. First we have to take it as fact and then we have to decide if we're patient enough to change; and if this is completed by sufficient enjoyment.
Swimming against tide OMG, you never should take that for average OW-times. Swimming forth and back the same distance, the way against takes much more time and drops your average much more than you'll earn on way back.
Don't miss to put enough enjoyment into your way!