several ways of doing this really. as your not a master of electronics i would sugest this as the best:
this will only count in decimals so you might have to measure in 100s of seconds
ok you want a clock first, somthing to set the time base for your counter. if you know how to make one a 555 Astable would be perfect, if not use a RC circuit. I've used a 555 Astable in my circuit i would think you want a time base of about 1second
The 7segment counter/ decoder powers the 7 segment displays, with "10" pin acting as the clock for the next counter decoder. as i say this only counts up in 1, 10, 100, 1000. you can keep adding an extra digit onto it.
if you really need a time format number (9:59) i could make a few changes to the circuit if you want?
Also i assume you want it so it activates when the gate is opend and stoped when the boat reaches the end? if you design a sensor arangment for that as well??
but why not just use a stop watch??