Hi Giles, I would have thought in this order.
1, 2 x 5/8
Bare in mind on the water there is little if anything in the signal path so I would have thought a simple 1/4 wave would work just fine and be the cheapest. I can open repeaters 20/30 miles away on 70cm with enough height with my 2/70 handy. The other consideration is the Kids handy's will will only have 1/4 ish wave antennas anyway so that could be the limiting factor in itself.