Battery Replacement Price and Waterproof(IP7 plus)
How much will it be to fix it at an Apple store in Malaysia currency and will it stay water proof or is there anyway to make it water proof again and how long will it take
iPhone 7 is IP67 compliant (see iPhone 7 - Technical Specifications - Apple). What that rating means you can check https://www.trustedreviews.com/opinion/what-is-ip68-ip-ratings-explained-2947135 and http://www.iphonehacks.com/2018/04/iphone-x-ip-67-water-resistant.html and plenty of other sites just googling for it.
You can buy a, Apple Certified case -should they exist- if you plan to use it under water to be safer.
About the battery replacement for Malaysia see iPhone Service Answer Center - Apple Support