The airfield floods at high tide, so you need to apply for permission first: https://www.aeroclub-avranches.org/docs-ext/demande_aeu_ad_restreint-bretagne_normandie_centre-vdl_2019.pdf and then get a quick briefing by phone (essentially they'll tell you if it's wet or dry). Tides can be looked up on https://horaire-maree.fr/maree/Avranches/. Much easier than it sounds. You might have to do a go-around to move the sheep off the runway. The clubhouse has been refurbished and is very nice, either to sit at the bar or outside in the sun. 100LL and mogas available when there's someone around. You enter your landing in an ancient leatherbound ledger that has all flights since 1933, including two by Antoine de Saint Exupéry. The aéroclub website has local accommodation and tours of the Mont Saint Michel listed on the 'Partenaires' page. Full writeup and photos: https://www.euroga.org/forums/trips-airports/9855-avranches-lfrw%23post_190400
Note: This information is for guidance only, with no warranty. Always consult the official flight planning sources (Notam, AIP, etc), and contact the airport directly to check latest pricing, etc.