You've got some good advice above on how to do it properly, and this is certainly the preferred way. You need to start saving your money
NOW and plan on a decent 120VAC install as soon as you are able.
Meanwhile, your "minimalist" approach could be made to work, providing:
1. that you learn a bit more about electricity and, particularly, about the power requirements of each piece of equipment
you plan to use; and
2. you provide at least "minimalist" protection for the power wire coming to the boat, in the form of a circuit protection device.
Both of these things are do-able with very little expenditure.
The items you list in terms of the amount of power they require range from negligible (alarm clock, printer, charging
cell phone) to heavy (hot plate, toaster oven
, heater). You need to pay CLOSE ATTENTION to the power requirements of each item. A heater alone can consume ALL THE POWER available from a 15A extension cord. Toaster ovens and hot plates can also draw a LOT of current
Write down the draw of each item, and learn which you could run simultaneously and which you couldn't. Try to stay considerably under the 15A maximum, because at 15A there will be some heating
of the electrical cord.
For at least some protection, you need a breaker on the extension cord. The 30A to 15A pigtail you mentioned is just an adapter and, it's very DANGEROUS if you don't learn something. This is because the dock outlet is wired for a 30A draw, while the extension cord will carry only 15A safely. However, if you plug
in more than 15A worth of stuff (e.g., heater and hot plate), you could be trying to draw a lot more than 15A thru the extension cord and the dock's circuit breaker wouldn't help a bit. You would cause a meltdown and probably a fire in short order.
The easiest way to mount a breaker in your situation might be to look for a power strip (multiple outlets) which has an integral circuit breaker for 15A. Then, plug
everything you intend to use into one of the outlets on this strip. If you plug in too much, the breaker will trip.
You'd also want to find the heaviest, most robust extension cord you can find, e.g., some of the ones used for construction.
Also, you want to be absolutely sure of clean and tight connections. When you plug the extension cord into the adapter pigtail, use good quality electrical tape to tightly tape the connection, both for physical contact and to waterproof the connection.
Understand that I'm not recommending you do this; only that if you absolutely plan to do a "minimalist" system like this you take the above-outlined steps to provide at least some protection for you, your boat, and the marina.