The investment must be substantial in proportion to the total cost of acquiring the marina; buying an established business at its price generally meets that test. Beyond the price, budget for working capital, the escrow and legal costs of the purchase, the consular application fee in the current schedule, and professional preparation of the E-2 file.
Substantiality is a ratio, and the purchase price sets the denominator
Because the couple is buying an existing marina rather than starting one, the cost of the enterprise is essentially the negotiated price, and paying it in full from their own committed funds makes the investment substantial almost by definition. The harder budgeting questions are elsewhere: working capital to carry the business through its off-season, the cost of the escrow and closing, any environmental or dock inspections a lender or insurer requires, and the professional fees for preparing a source-of-funds chronology and business plan. The consular visa fee is set by the State Department and should be checked at the time of application.
If part of the price were financed by a loan secured against the marina's own assets, that portion would not count as the couple's at-risk investment; a loan secured on their personal assets would. Keep the structure simple unless there is a reason not to.