The U.S. Navy bought two very large flying boats during the heyday of the type. Both were originally conceived as patrol aircraft, but were repurposed as large transports and were only purchased in small numbers.
The first of these was the Martin Mars, which first flew in 1942. The Mars, originally the XPB2M patrol plane prototype, became the JRM and was used for several years as a transport between California and Hawaii. Only seven were built. It was powered by four R-3350 or R-4360 radial piston engines. The 4360s, while more powerful, proved less reliable and were ultimately replaced by improved 3350s. The JRM served with the Navy from 1944 to 1956. After retirement, they found a new life in Canada as firefighting water bombers and gave distinguished service from 1960 to 2024.
https://en.wikipedia.org/Martin_JRM_Mars
The Navy did not give up on large flying boats and a second generation appeared. This time it was Convair that designed a flying boat of about the same size as the Mars but took advantage of technical advances in aerodynamics and propulsion. The resulting XP5Y-1 was powered by four large T40 turboprop engines. Unfortunately, the T40 proved to be unreliable, although the Navy decided to buy a dozen aircraft in transport form as the R3Y Tradewind. Like their JRM predecessors, they primarily flew between California and Hawaii. Several R3Ys were also outfitted for aerial refueling. Although the R3Y set a number of records, it also was retired prematurely. By this time, transoceanic travel via landplane was also proving practical, contributing to the end of large flying boat operations.
https://en.wikipedia.org/Convair_R3Y_Tradewind
Just for some context, both these flying boats had a maximum takeoff weight of about 165,000 pounds. The Navy's standard patrol plane of the 1960s-on -- the Lockheed P-3 Orion -- had a max takeoff weight of 135,000 pounds. The current Navy patrol plane, the Boeing 737-based P-8A has a max takeoff weight of over 189,000 pounds and has more range and is much faster.