As has been said, you will want a throrough picture of the competition in frozen fish dinners. From a marketing and sales stand-point you will ask these questions: 

• How much do competiting products cost in the store? 
• What are their types, varieties, sizes of fish dinners? 
• How will your product stand out from the current products? 
• Who are the target markets for existing fish markets? 
• Who is your target market? 

One way to find out who is buying fish dinners is to look at the way the existing products are being marketed. 

When you think about production there is a new set of questions you will need to answer. 

• Where will you produce your fish dinners? In what kitchen, factory, etc.?
• What products will your dinners include?
• How much will each dinner cost to produce?
• Where will you sell the fish dinners?

In a nutshell, you should consider the risk involved in starting a new business. Is there demand for your product? Is the demand sufficient that it will sustain a price which will return a profit for you? Will you operate the business yourself, or will you hire employees? Should you form a corporation, a partnership, or operate as a sole proprietor? Are there licenses, taxes, etc. that must be taken into account?  You should be able to answer all these questions AND be comfortable with the responses before you enter into any type business.

Supposing that the "regular" market for frozen seafood is saturated, you might want to consider what could distinguish your product from others.  Is your seafood from an environmentally conscientious source? Is your packaging "green"?  Does the preparation focus on healthy cooking?  You could probably appeal to various niches if your product stands out in one or more of these ways.  You would need to do some research on what the present market is, for run-of-the-mill frozen seafood and then do some research on the potential market for "green" consumers and perhaps on vegetarians, who do sometimes eat fish and who are generally more conscientious consumers. 

I agree with number 3 above.  Your first step is to determine your own niche and see where it fits within the scope of the current market.  I would think there would be a better avenue for a more gourmet meal with fish as a main entree than just selling fish.

You would want to know how many name-brand competitors there were, and what their quality and prices were like. You would also want to know what price and quality the big retailers sell their store brands for. It may be that the market for low-priced fish dinners is saturated, but there might be a market for better quality products, sea bass fillets instead of fish sticks, for example.

The main things that I would want to know are how many competitors were out there and how much of a market we would be competing for.  I would want to know how many two-income families there are in my market area because I would think they might want frozen dinners more than other families.  I would want to know what other kinds of demographic groups eat this sort of food.  I would want to know how many people from those groups were in my area and how many other companies (with how much capacity) were selling products that would compete with mine.