I regularly fly from Dominica to Barbados - 177 miles over open water in my Archer II - couple of times a month; VFR here is max.8,500 in Barbados airspace - generally, I fly west at 6,500 and east at 5,500.
One thing is that the water is warm - have life jackets worn by all, a four man raft and a GPS portable locator beacon.
Would think that the biggest issue if one had to ditch on your passage would be cold, so maybe a couple of immersion suits would be a good idea as one would maybe have to be in the water for hours.
Need to make sure that the airplane is well maintained and FULL of fuel - if the weather descends you would want the time to fly out of it.
Of course never do the over water thing immediately after maintenance, fly a couple of hours over land to make sure no problems remain.