I agree, he is such a nice, gentle man (as well as a gentleman), it is a little difficult on first glance to connect him with the sea. There are a few reasons why this approach would be useful to him though, imo

1) A ship would involve working in very close conditions with a large crew. It is of vital importance not to let petty disagreements or personality clashes to get in the way of the main task. Glossing over the huge amount of looking glass at Kellynch seems to fall into that category. The Admiral is sharp enough, imo, to pick up on what that would imply about a man's character. After all, he does mention it. He simply chooses not to get mad about people's foibles. 'People are people' seems to be his approach.

2) Thinking the best of people tends to get the best from them, in my vh experience. The Admiral includes the good in his summation of Benwick's character, as well as acknowledging his faults, the implication being that is the best that can be thought of him after reasoned consideration. It would of course be important in the life-or-death situations of the 18th century navy to have an accurate summation of someone's strengths and weaknesses. There isn't a blind ignorance of Benwick's potential to make mistakes on the Admiral's part, but neither is it a blind ignorance of his real strengths. He will have to bring these strengths out in people to actually succeed at war, encouraging people to suggest ideas, stratagems or point out flaws without too much fear of being dismissed. Such a balanced, almost avuncular attitude of the Admirals will inspire confidence in that regard.

3) Sailing does tend to require an ability to deal with each emergency as it happens. Looking too far ahead could paralyze one with fear. Perhaps the Admiral has an ability to live in the 'now' rather than giving everything a deep analysis, which could take up a lot of time and energy and not necessarily achieve very much. This gives him a naive aspect on land; at sea he might have a calm ability to channel most of his mental energies on staying alive in the next day or so. I don't actually think it necessarily makes him naive; he just has an ability to avoid being narcissistic dwelling on what might make him feel insecure. Insecure people don't make very good naval captains. Look at Captain Ahab.

4) Having lived in such a life-or-death situation for so long, he is bound to have an adjusted sense of priorities. After all, no one has died due to Captain Wentworth's romantic entanglements. One Henrietta or Louisa is just as interchangeable as each other, in the sense that they would both make a nice man equally happy. (He probably isn't used to being around too many young girls anyway). I suppose the war being over for the present, the Admiral is approaching his life during the novel asa form of extended holiday or period of early retirement. Without his usual sense of heavy responsibility, everything is more of a joy. He has probably not had much opportunity in the past to get a long period of leave with his wife and family, and has looked forward to such a time for years. He has been active, and is now taking a rest, unlike Sir Walter, who has made inactivity his life's work and so is actually vaccant rather than simply relaxed like Admiral Croft.