That seems like an unusually high amount of spawns but for your first swamp experience I don't recommend you sail straight into the swamp and instead cross into it by foot from a black forest / meadows biome to get used to the mobs.

Also the grim reaper flying bro spawns at night so don't be there at night until you can handle yourself. All of the mobs can be parried and easily beaten once you are comfortable but first time........ I still remember mine and it was a similar experience to yours. Go slower