[Nyororin]

Yes, I agree - say it is an allergy. If you make it clear that you are allergic to shellfish and that there may be other sea food that you have a reaction to, I am sure that a family will work to accommodate you. I wouldn`t expect to be fed meat on a regular basis, but vegetarian food is likely not going to be much of an issue. It depends on area though...
Around here, it seems like fish is a 3 or 4 time a week thing... With meat 2 or 3 days and maybe no meat or fish another day. But up around the in-laws, they eat sashimi even for breakfast literally every day. I don`t think I`ve ever seen a meal (short of curry) that didn`t include fish.

So if you`re heading to a coastal town things may be very tough to pull off.

Beef is probably NOT something to expect or ask for as it is pretty expensive - chicken and pork is the way to go.