Sorry to totally bore you with rules and regs but this one is worth considering, if you lift a boat on slings then you should either attach the slings to a set of brothers (long chains which mean that the angle created by the slings is 90 degs or less as loads increase inwardly onto the hull exponentially the further you go above 90 degs.) or you fit a set of spreader beams just above deck level to take the compression load. If this is greater than 90 degs you risk damaging your boat due to compression loading.
Again I must say I would not work under a slung load whether it is suspended by hydraulics or by mechanical lifting device such as a chain block or pull lift as the slings can easily slip espescially forward due to the hull shape.
Believe it or not lifting is the biggest killer in the north sea and should not be fooled around with if you are unsure then you should seek professional advice.
Hopefully not bored everyone to tears