I used to windsurf and sail in a 3mm, shorty in the summer, full suit in winter on the UK coast, also used to go swimming for around a 30 min mile in just shorts in summer, water temp about 60F summer and 45F winter. I wouldnt however go diving in a 3mm full suit in less than 60F and i have just bought my 1mm suit for marine protection rather than warmth when the water warms up to around 80F in the summer.
As for difference, i was only in the water for a limited time sailing and windsurfing, but there was always the sun on my back to keep me warm. Underwater is all around heat sink, i could dive in a thinner wetsuit in cooler waters, but would perfer to be a little warmer.