Diploma

You don't need any of these institutes.
Institutes like these wouldn't make much of a difference while you apply for an airline.
Every airline has their own training, and they don't care or give any additional priority if you have been in one of these institutes. Airlines have about 4-7 weeks rigorous training, with highly qualified instructors.
It's simply a lot of money for no good reason, the hype around these institutions are simply ridiculous.
You rather focus on being the perfect candidate for the job, and this can absolutely be done at home. We have the Internet to get all the information in the universe that we need, and trust me, that's all we need.
It is better you don't go for any institute first, attempt some interviews.

Yes, you can do Diploma in engineering after class 10th. Most of the colleges offers 3 years Diploma courses in Engineering and after that u can go for B.Tech also as lateral entry in same college or other too. Also, after class 12th You can appear for various entrance examinations like JEE, and other entrance examinations conducted by state universities and other private universities. If you qualify in those entrance examinations, then you can appear for the counselling process, where you can choose your preferred branch (Marine engineering) in a college of your choice. I hope it helps. If you have any further questions, kindly revert.

T is indeed very nice to hear from you that you are interested in doing your Diploma in electrical engineering after completion of your class 10th in 1819 session. Yes I think you should be able to get your desired branch with this percentage which were obtained in the 10th class. Normally speaking all the government or government aided polytechnics admit students based on the marks of 10th in majority of the states and this process is coordinated by the Directorate of technical education. Hence I would like to advise you that please go to the website of directorate of tech education of your state to know the complete details. Process of admissions invariably begin after the publication of the results of 10th. Best wishes to you.