Tips on how to format a manuscript in marine biology and physiology
The general formatting advice from @CapeCode is sound. Generic font (certainly not anything odd) and 1.5-2 line spacing is fine for the text. Tables and figures should go at then end. Many journals ask for figure files not to be included in the manuscript file at all, follow such advise. If figures are separate all figure captions go at the end of the manuscript file. 

Regarding line numbers it is not a major issue unless the journal specifies something special. Some manuscript systems provide only PDF-files of the manuscript. Some reviewers then prefer to have the line numbers to locate their comments. If the journal provides the Word files then as @Cape Code states these can be more of an issue. So try to see if you can figure out what the journal sends out but in the end I do nto think it is a major issue.

In addition, I strongly want to push following any instructions for authors to the point. Make sure your use the proper reference formats and follow other journal formatting details. If no explicit instructions for authors exist then look at recent articles to see what styles the journal uses. A manuscript that follows the journal style looks more serious than a manuscript that does not.

As for length, I would say that 6000-8000 words incl. references is a reasonable size for a regular research paper in your disciplines, shorter is possible but longer should be treated as a warning. If you really want to know just count the words on a page and guestimate the total for the article, you will not be far off.