You might want to get a triathalon suit – as swimming is (presumably) part of their design criteria (so will be designed to allow good an easy arm movement). (I would suspect that most 'wetsuits' aren't designed for proper swimming – winsurfing, diving, surfing etc))
Also check fitting type – personally I don't like velcro or stretch and use zip – however if the zip rubs you might be unhappy. Esp how it interacts with shoulder and arm movements, as you'll be doing a lot of them. Seam type is also worth understanding (but probably not too relevant to summer usage) – but for winter you want a waterproof seam to minimise flushing.
Would a shortie do (short arms, short legs) – I use that to windsurf/surf in summer. You also get convertibles (addable arm/leg lowers etc)
You probably want a 1 or 2 mm (5/6 is winter) A 3/2 means that the body part material thickness is 3mm and the arm/leg part is 2. The thicker is warmer (* also depends on quality and exact type )and harder to move as it exerts more 'force' when you try to move it. ie you don't notice a rash vest you DO notice a full winter suit with a chunky shoulder back zip.
Try searching for rash vest – – it's about a .5/1 mm T shirt style top (£25 ish)- would that help. I use it as a base layer in winter but you seem people surfing them in summer)