Brittany is a lot cooler than Malta, full oceanic climate, and about 48 degrees North. For diving, water in summer is about 15-16 C where I dive, but it is the coldest place (at Atlantic/Channel limit). More south it should be better. In summer, I drop the shorty, and take only the full 7mmm (wet) plus a hood, diving socks or boots and gloves.
On the beaches, water is warmer, thanks to the heating effect of the tide (sand warms at low tide, water runs on warm sand and warms).
On the beaches, water is warmer, thanks to the heating effect of the tide (sand warms at low tide, water runs on warm sand and warms).