library(marmap)
library(ggplot)

greys <- c(grey(0.6), grey(0.93), grey(0.99))

# Get map data
bat <- getNOAA.bathy(-68, -51, -48, -39, res = 1, keep = TRUE)

autoplot(bat, geom=c("raster", "contour"), colour="white", size=0.1) +
  scale_fill_gradientn(limits = c(-6600, 0), colors=c("steelblue4", "#C7E0FF")) +
  NULL

# I have tried setting different limits in `scale_fill_gradientn` but without much luck

# Generate map
autoplot(bat, geom=c("raster", "contour"), colour="white", size=0.1) +
  scale_fill_gradientn(limits = c(min(bat), max(bat)), colors=c("steelblue4", "#C7E0FF", greys)) +
  NULL

# Baseplot
plot(bat, image = TRUE, land = TRUE, lwd = 0.1, bpal = list(c(0, max(bat), greys), c(min(bat), 0, blues)))
plot(bat, lwd = 0.8, deep = 0, shallow = 0, step = 0, add = TRUE) # highlight coastline