Hey !
Bsc Maths focuses primarily on developing mathematical skills in algebra, calculus and data analysis. A Higher Secondary degree or equivalent is needed to pursue Bsc Maths.Upon graduating in this stream, one can pursue courses such as  Msc (Maths, IT) ,can also purse MBA, also can  take up teaching as profession.
The average salary offered to postgraduates lies in the range of 3-5 lakhs per annum.
Marine engineering Career development opportunities include aspects like equipment procurement, marketing, and sales. You can choose this course if you have the below listed interests,
If You  have interest in Marine engineering and its related fields.
If you have good interpersonal and communication skills.
Candidates who want to become a ship manager.
Average salary varies for every job you take up...for example if you work as a port Manager then you can earn an average of  14-15 lakhs per annum,  Marine engineer (7-8 lakhs per annum) and so on.Choose the field According to  your interests and other considerable factors..both of the fields have their own importance.
Hope this helps
All the best !