We present here an experimental study of Wigner islands formed by electrons floating over helium. Electrons are trapped electrostatically in a mesoscopic structure covered with a helium film, behaving as a quantum dot in the near-classical limit. By removing electrons one by one, we are able to find the addition spectrum, i.e., the energy required to add (or extract) one electron from the trap with occupation number N. Experimental addition spectra are compared with Monte Carlo simulations for the actual trap geometry, confirming the ordered state of electrons over helium in the island