This is the approach I would start with. You can make the seamine bob up and down with a func_bobbing if you want, but then you need to have a little different approach to it. This example would just be a static one.
For example make the actual visual seamine into a func_static (other possibilities too). Give it:
(key value)
targetname seamine_1
Then make a target_effect that will be the actual explosion. Play with the values and boxes to tick. You could start with something like:
(key value //comment)
targetname seamine_1
type metal
mass 100 //amount of debris... try out yourself
dmg 400 // damage radius... try out yourself
spawnflags 5 //ticked the boxes tnt and smoke... try out yourself
Then make a trigger_once, which will trigger the explosion and hide the visual seamine. Give it:
(key value)
target seamine_1
So basicly the trigger_once targets the func_static and the target_effect.