Know are different worlds. According to most if not all OB's manufacturers literatures which have read states that the best prop thrust is achieved while boat with well deck weight distribution rides parallel to water level when OB is trimmed to 90?. Under those conditions, prop shaft, anticav plate and hull will ride parallel to water level too. Be aware that we're talking about recreational standard boats driven with standard manual trim OB's usually delivered with 3 blade props.
On that perfect conditions prop angle of attack will receive even (harded) incoming water passing under middle hull. On my last angle transom modification under (I swear...post) have gained 200 rpm reduction at wot with same speed than before while OB was sitting not at ideal 90? due to having factory transom deliverd with a miss shape issue. Now sitting at 90?, with lower leg sittting at the sweet ideal height has contributed too to achieve full prop thrust at full max cowl's HP.
What has prop thrust to do with aviation ? Wanted to ask, know is : when plane has reached its flying level say 15 K feet and plane is fying with artificial horizon indicator right at middle line, would you say that prop is achieving its max air thrust propulsion independently of blade pitch used for that altitude ? In other words, does each balde receives the same amount of incoming air when plane flies horizontal leveled ?
Pusher being a licensed pilot shoud have a proper tech answer for that.
Happy Boating