Hi Mattia,
If what you do is mostly snorkeling, I recommend you the RX100. But if you are more interested in macro photography, the OLY em5 in a Nauticam Housing with the dedicated 12-50 port is the best option-value for your money. Semi w/a and very good macro quality, everything in a single port.
It all depends on what are really your preferences, because if you travel without Strobes, the difference in size is not that big.
