Many yacht operators offer a provisioning service so if you’d like your shopping done for you ready for your arrival, ask if this is available.
You will usually be given a list to tick or mark quantities on before you travel and you will usually pay the shopping bill plus a handling charge. Where items are unavailable you make get substitutes which may or may not be ideal! Where international brands are available you will usually be supplied with these in preference to local equivalents. This can make a big difference to the bill as price differences are sometimes much more marked than between brand names and supermarket’s own brand in the UK.
You may be surprised how much is available these days. I’ve even found Marmite available in Turkey. So I think the most missed item that you may wish to take with you is your favourite brand of tea. I’m not a tea drinker so I don’t understand why no one on earth seems to like Liptons tea (obviously this isn’t true as they wouldn’t still be around but there seems some social stigma about admitting to it)!