Unless you're transporting persons or property from A to B for hire, you shouldn't have any requirement for a 135 operation, as there are no provisions in 135 that would allow you to do what you propose.

Dropping cargo and returning to base isn't a 135 operation. For a Seneca, Part 125 doesn't apply. Dropping buoys, if no chemical release is associated, likely won't have any need of a 137 certificate. The previous poster who dispensed economic poisons (baits) was exempted from Part 137 by 137.11(c), so long as using a public aircraft).

If instead, the flight also intends to transport persons or property from one base to another, the flight may fall under 135, unless it's a public use aircraft. In most cases, however, where a contracted airplane is used, the government insists on 135 certification if its personnel are carried aboard or transported.