Aquaculture is different from growing in the ground period. Lettuce grows faster for sure, but whether in the ground or in a aqua bed the spacing requirements will be the same.
Manoa is a mini head so it is usually a smaller lettuce. Larger lettuce varieties would have wider spacing. You want enough room for the leaves to grow and be just touching when it is time to harvest. Lettuce will adapt to its spacing. If you give it less space, the heads will not spread out as much and people who take the outer leaves off as needed often plant closer. Tip burn resistant lettuce is better for summer. In summer I usually grow red lettuce like lolla rosa which has better tip burn resistance. Manoa is actually a tip burn resistant lettuce just not as much as the red lettuce. I only grow romaine in the cooler months, it doesn't like heat at all, becoming tough and bitter.
In summer the lettuce grows much faster and matures in half the time, however, the fast growth can cause tip burn which is a physiologic disorder. The lettuce grows faster than the plant's ability to transport nutrients specifically calcium to the growing tops. When that happens lettuce needs to be harvested before it heads up. Giving extra calcium doesn't help because it is a relative deficiency not absolute. Even when there is enough calcium in the soil or the plants are sprayed with foliar calcium, tip burn still happens.
In the cooler months I plant my lettuce out in full sun in my own gardens. During the summer, I plant lettuce under the citrus trees and between taller plants as they do better than in full summer sun.
My favorite lettuce is butter lettuce. They are large, heat resistant, but they bruise and wilt easily. But very nice and tender for sandwiches.
Happy gardening in Hawaii. Gardens are where people grow.