Seafood should not be refrozen even if they have ice crystals. These foods may be cooked and then refrozen if the temperature of the food was colder than refrigerator temperature. Do NOT cook if they have warmed to room temperature. You cannot always smell or see all signs of spoilage. Unfortunately, you may not know how warm thawed food has really become, so be overly cautious and do not take chances.
Fruits lose their shape and can become mushy when thawed.
They ferment when they spoil. This is not particularly dangerous, but the taste or texture can become unacceptable. Fruit can be refrozen if the taste, smell, and texture are acceptable. Using them in cooking or making sweet spreads are a good way to use these thawed fruits. Ice cream and other products containing air do not result in an acceptable product when refrozen.