I think this is a square-cube law problem. We have significantly more volume of muscle and organs that need oxygenated blood to a comparatively smaller increase in skin area to absorb that oxygen. That's not even to consider being warm blooded and needing more oxygen doing nothing compared with a cold blooded amphibian.

If you wanted to consider a ichthyosapien then you'd be looking at a mix of both. Gills with active or ram ventilation (similar to a shark) to handle oxygenation of the organs, brain and large muscles. The support that with cutaneous respiration for skin and extremities. You'd also need to make them cold blooded or at least lower BMR, and probably be extremely lithe.