Master level fisherman villagers trade 1 boat for 1 emerald. Generally, the type of boat they trade for is dependent on the wood type which grows in their biome. For example, plains villagers buy oak boats and taiga villagers buy spruce boats. However, desert villagers consistently buy jungle boats, even though jungle trees do not generate in the desert. Similarly, swamp villagers buy dark oak boats even though dark oak trees do not generate in the swamp.

To reproduce, type in the following commands:
/summon minecraft:villager ~ ~ ~ {VillagerData:{type:"minecraft:desert",profession:"minecraft:fisherman",level:5}}
and
/summon minecraft:villager ~ ~ ~ {VillagerData:{type:"minecraft:swamp",profession:"minecraft:fisherman",level:5}}

This issue/discrepancy could easily be resolved if swamp villagers sold oak boats or mangrove boat and desert villagers sold either oak or acacia boats.