International and state borders in water create issue determining centroid.

I am able to get the center latitude and longitude of each US county and Canadian census geographic unit (CGU). However, there are some false counties that arise through the process, particularly in the Great Lakes region. In this region, there are some random polygons within the Great Lakes (state/international borders) that produce these false counties. Is there a way to remove these aquatic polygons so that I only get the central latitude and longitude of the land polygons?

library(ggplot2)
library(dplyr)
library(geodata)
library(sf)

# Gets the polygons of each US county and Canadian CGU

aa <- gadm(country = c('CAN', 'USA'), level = 2, path=tempdir()) %>%
  st_as_sf()

# Gets the central latitude and longitude of each county and CGU

centroids_aa <- aa %>% 
  st_centroid(of_largest_polygon = TRUE)

# Map of centroids (there are obviously incorrect counties and CGUs for sites within the Great Lakes) 

ggplot() +
  geom_sf(data = aa,
          mapping = aes(geometry = geometry),
          color = "black") +
  geom_sf(data = centroids_aa,
          mapping = aes(geometry = geometry),
          color = "black",
          fill = 'red',
          shape = 21,
          size = 2) +
  coord_sf(xlim = c(-93, -81.5), ylim = c(41.5, 50), expand = F) +
  theme_bw() +
  theme(plot.margin = grid::unit(c(2,2,0,2), "mm"),
        text = element_text(size = 16),
        axis.text.x = element_text(size = 14, color = "black"),
        axis.text.y = element_text(size = 14, color = "black"),
        panel.grid.major = element_blank())

link to Stack Overflow question on the same issue.

rspatial / geodata

International and state borders in water create issue determining centroid. #47