Hi @cmazoyer,
croco_pytools/xcroco can perform vertical interpolation of any CROCO variable here is how:
I assume libraries are imported such as in tuto_xcroco.ipynb,
# create the grid
drop_variables = []
ds = io.open_files(
croco,
gridname,
filenames,
grid_metrics=2,
drop_variables=drop_variables,
chunks={"t": 1},
# chunks={'t':1, 's':1, 's_w':1},
)
ds1 = ds.sel(t=seldate, method="nearest")
grid = gop.fast_xgcm_grid(ds1, croco, grid_metrics=2)
ds1
The appropriate function to perform vertical interpolation is gop.isoslice()
The naming is a bit funny because we should be able to interpolate variables onto another coordinate than depth (ex: density) but this feature is for the next release 
First we need to get vertical coordinates of the variable we want to interpolate. There a function gop.get_z() for that.
And here is the action:
depth = -1
# depths at rho-points
z_r = gop.get_z(croco, ds=ds1, z_sfc=ds1.z_sfc, xgrid=grid, hgrid="r")
# vertical interpolation
z_temp = gop.isoslice(ds1.temp, depth, grid, target_data=z_r, axis="z")
kwargs = {"vmin": -1, "vmax": 1, "extend": "both"}
cplt.plotfig(z_temp) # , **kwargs)
I created a branch 103-croco-basic-operators with a notebook xcroco/tuto_operators.ipynb
Hope this helps,
Jeremy