Eric,
There are no "SESI" or "SES/MES instructor certs." For one, you're describing ratings on the certificate, not certificates. Two, your expired instructor certificate with its associated ratings (ASE, IA, AME, commonly referred to as the CFI, CFII and MEI) already include those privileges if you have ASES and AMES ratings on your pilot certificate. I.e., there are no specific seaplane flight instructor ratings.
You can add a new rating onto your expired flight instructor certificate that you don't already have, such as glider (?), which would also require a glider rating on your pilot certificate. But if you don't want to do that, you'll have to reinstate your CFI, which is treated no differently than the practical test for the flight instructor rating you'd like to reinstate. Completing the requirements of 14 CFR 61.199 reinstates all of your flight instructor privileges, so for example, if you completed a CFI-IA reinstatement, your CFI-ASE and CFI-AME privileges would also become available.
Hope this helps.